If it's a Faerie regio, Arcadian travel might work to get you in/out of it, but that's not going to be useful for most magi.
Lot of experience with this, since a key part of my current saga is about Regios.
I do want to comment that there is a distinct difference between a Regio which has its "known" entrance path/ritual destroyed and one which is sealed. With the former all it most likely takes is a spell like Piercing the Faerie Veil (ArM5, page 158) of the appropriate Realm or an appropriate supernatural virtue/ability while with the later it is pretty much limited to some form of teleportation.
There are multiple ways to travel between the levels of a Regio and following a path or (non-magical) ritual is only one of them. While this (and its less effective form of getting "lost") are the only method open to mundanes, Magi or those with certain supernatural virtues have access to the best method. If you look at page 189, you will find much more information.
Magi who dwell in a given Magic Regio can themselves travel between the levels (including leading others) without any spell or ability. They just have to dwell in it long enough to be considered native. How long that takes is up to your saga, though it should not be overly long. A season perhaps? Any supernatural being (Might or affiliated) of the appropriate Realm and native to the Regio is the same. The Hermetic Cats in our Regio often lead the mundanes between the different levels (other than the 1st which is sealed).
Beings not native need some way to see between the boundaries. For example Magi who do not dwell in a given Regio generally need a version of the above mentioned spell appropriate for the Realm associated with the Regio and do not require any rolls if the spell is cast successfully. Second Sight and Magic Sensitivity are also effective, though they require a roll. Sense Holiness & Unholyness works for Divine or Infernal Regio.
Granted it is possible that natives might not be able travel this way in a specific Regio, but they "almost always enter and leave freely". Ones that do not allow natives (and those who can see the boundary) to travel across are 'Sealed'. If there was a way into them it was more than a path, generally some type of portal or gateway. With these if that portal or gateway is destroyed than the only way out is generally teleportation or the creation of another portal or gateway.
In our saga we specifically sealed the 1st level using Hermetic Architecture, meaning the only way in or out of it is through a gateway or teleportation. There are multiple gates, one to the mundane and two to deeper levels (plus a secret gateway only the Magi know about). This had the effect of destroying the normal "paths" into that level. If something would happen to the gateway to the mundane, it would trap everyone in our Covenant without access to teleportation. We know this and so have several backup methods which range from varies teleportation charged/enchanted items (along with AC) and several portals which we normally keep deactivated.
There is a 4th edition adventure with exactly this plot. The players visit a covenant where one of the magi went missing after the entrance to the regio was closed (it only appeared when the Dominion subsided, but the Dominion got stronger due to a relic being moved to the place where the entrance was).
The way the adventure is written, he only escapes with external help. But I wouldn't consider this as a canon example for all regionnes. Only something that might happen.
Strictly speaking, native relates to birth ...
Time to break out the fertility magic...
Or just, you know, set up a midwifery "shop" in the regio.
Ars Magica 5th edition, page 189
Any being native to a regio and part of or affiliated to the realm can lead any number of characters to any level of the regio. Magi are affiliated to the Magic realm, and if they live in a Magical regio they count as native to that regio.
Per the book, Magi only have to dwell in a Magic regio to be considered native to that regio. While I was suggesting maybe that should be for a season, the way it is written the moment they begin living in the regio they become native.
I don't really read that or using Pierce the [Realm] Veil or using Second Sight/Magic Sensitivity/Comprehend Magic/Sense Holiness & Unholines/any other Sup abilities as "this person can cross anywhere" but that these things allow them to find the path or actions needed to cross which might require walking around the whole thing to find where crossing is allowed. This does not remove the possibility that some regios are more permeable and uncloseable without something like the Stasis power or use of Hermetic Architecture to seal it off but that many will be sealable by rather mundane means (destroying the Gate, collapsing the tunnel, etc) if you know where all the entrances are.
EDIT TO ADD: And nativity I see as granting the knowledge of where these paths are and what actions need be taken due to common use and, again, not making the whole border permeable to these folks.
That is fine if you want to house rule it that way, but you need to go back and read the section I keep pointing to since it does not agree with you.
There are two ways to enter a regio voluntarily, and either or both may apply to a particular regio. The first involves seeing into the regio and then crossing the boundary. This requires some supernatural ability. Second Sight allows a character to see into any regio, as does Magic Sensitivity. Sense Holiness and
Unholiness allows a character to see into Divine and Infernal regiones.
...Anyone who can see the next level of a regio can lead any number of people into it.
The second way of entering involves following a particular path or ritual.
Seeing into a regio is a different way of entering it than following a path or ritual.
I did go back and read it before making my post and, as it and you stated, not all regios allow for all types of entry and I would expect relatively few have the "this border is completely permeable at any point by accepted crossers" option of crossing into the regio.
Actually it would be the other way around. The entire section on Entering and Leaving gives means and it is not until the last paragraph that it says there are "some" regios which are exceptions. The only part of the last paragraph that does not apply to a minority of cases is "Natives of a regio can almost always enter and leave freely". If you want to play the majority of regio are of a more restrictive entry than you can, but that would be a house rule.
The majority of regione are of no relevance whatsoever. Any given saga will only ever visit a minority of the regione in Mythic Europe. Hence any argument about what is most common is moot.
Entering and leaving freely can include knowing the paths but you can say the rules are more strict and nearly always allow total permeability. YSMV.
Indeed. When I say that people are free to leave the room, I always mean to imply that they are welcome to leave by the open door; I would not imply that they are free to break a window to leave by the backyard.
When you are answering someones question about the rules with something other than YSMV or "in our saga..." it is. The proper answer would be "this is how the rules say it is done, with the option for exceptions". Answering as if the exceptions are the norm and anyone doing it by the book is very disingenuous.
What I am saying if that "entering and exiting freely" will mean the door is open, as Loke put it more eloquently than I did, and not that there are no walls and both meanings are perfectly legitimate there and I didn't see anything in the rules I read that explicitly pushes one reading of that over the other.
I've always read "natives can enter and leave freely" as the book making it clear that residents of a regio are usually not bound to that place. It's there not to specify how they can leave or enter, only to make it clear that they are not prisioners.
First phrase of the third paragraph: "There are two ways to enter a regio voluntarily, and either or both may apply to a particular regio."
Actually, you quoted it yourself. Here:
Considering that there are published adventures where a covenant is trapped in a closed regio, I'd have to say that magi living in a regio do not automatically get to universally leave however they wish to do so.