Folk witch longevity rituals: clarification needed!

No, I am not saying that. I am saying that sometimes they are "optional", in the sense that anyone part of the magical tradition can carry out some activities, but characters with a related ability might add it as a bonus. For example, every folk witch knows how fix arcane connections, but only sahirs versed in Solomonic Astrology do. Every sahir knows how to initiate other sahirs in the Arts he knows, but if such a sahir is also good at Solomonic Travel, it's much easier - in this case Solomonic Travel is optional. Ideally, it should always be stated when supernatural abilities are optional and when they are not; the sahir mechanics do it, the folk witch mechanics do it for pretty much every activity except longevity rituals and familiars - that's why I asked for a clarification!

Not really, it depends. A Gifted, intelligent individual can help an Hermetic magus in the lab as soon as he has a score of 1 in MT, no need to open the Arts. An unGifted mundane who has undergone her first initiation as a folk witch, and has gained a score of 1 in MT may fix arcane connections (something a sahir needs Solomonic Astrology 1 to do) and perform any other folk witch activity without explicit requrirements, like analyzing potions.

No. To be a folk witch, you must have either your Gift opened, or you must have undergone at least one initiation. Now, suppose you have only learnt how to fly a broom, and your MT is 1. Can you bind a familiar? That's unclear, but assume the answer is yes, in principle. Ok, you need to match 10+its might+its size with Int+MT(1)+Aura. So, well, yeah, at this point you can maybe bind a non-magical cat (size -3) if you are pretty smart and in a reasonably strong magic Aura (Int+Aura at least 6)... and have already befriended the animal (with which you can't speak, so at the very minimum it will be pretty hard to get him learn MT - no help in the kitchen from it). Sooo... hmm... now you have a True Friend cat with Int-3, but somehow I fail to see how you've broken the game.
What if we say such a young apprentice can theoretically create a longevity ritual for herself? Well ... if she's 10, she might pull it off (she can use 2 pawns of vis with her MT 1), and gain a bonus of 1, possibly 2 (int+Aura equal to at least 5) to her subsequent aging rolls until she fails one. Of course, if she's 11 or older, she can't pull it off, as she'll need 3 pawns. Again, it does not quite feel world-breaking, or inappropriate from a flavour point of view.

It may be ridiculous, but it's canonical. Because that part of Solomonic Travel has been integrated in the broader framework of the sahirs, unGifted Solomonic Alchemist can initiate others in Solomonic Alchemy, using a method that benefits from, but does not requrie, Solomonic Travel. Similarly, inventing naranjs benefits from Solomonic Astrology, but explicitly does not require it.

No, see above. Though I do agree that situations in which a supernatural ability benefits, but is not required in, a magical activity are uncommon.

How so?
Unless I've missed something (quite possible), for each lab activity involving an Art, for sahirs it's always stated explicity whether that Art is required or not. This is good, because there's no need to know what the "default" is. And it makes my observation correct. To prove me incorrect, you should find a lab activity that does not make explicit the need of the relevant Solomonic Art(s).

Take "Vis Manipulation" (p.38). First, it states that you explicitly need Solomonic Alchemy to extract vis from a supernatural aura. The second paragraph talks about extracting vis from a supernatural being or enchanted item. The fourth paragraph talks about vis transfer. Neither the fourth paragraph nor the second explicitly say you need Solomonic Alchemy to perform those actions, but use that as a lab total. I think it's somewhat ludicrous that you can't extract vis from an aura without the ability, but you can extract the vis from a magical item or a spirit just fine. And I don't quite think Vis Transfer is quite the same action either - but it does use a lab total regardless. Bindings don't explicitly require the art per the text, but there is no contrarian indication - that you don't need it either, even while it does go on to say that binding spirits can be done if you couldn't open the item with Solomonic Alchemy. My reading of the chapter is that it is more systematic in saying when you don't need something than it is in saying when you do.

Lab activtites like learning spells (naranjs) that are not summoning spells?
Or create a lesser binding?
Or create a warding?

As far as I can tell none of the above are explicit about the need or lack of need of the corresponding Solomonic Art.

And looking at the Intellego and Vim descriptions on p.77 of the corebook, or at the InVi guidelines, or at the part that describes how a Lab Total is generated, it also doesn't say that the magus Intellego Vim Lab Total is used to Open the Arts or to investigate an enchanted device. Nor does the descriptions on Creo,
Corpus or the Creo Corpus guidelines give any indication that an hermetic magus uses his Creo Corpus Lab Total to create a longevity ritual. Each of these details is given on the moment the lab activity is defined.

Witches explicetly use their Healing Ability Lab Total for their longevity rituals.

It might be pedantry, but, note that this isn't a "Solomonic Travel Lab Total" but an "Opening Total" based on Magic Lore Lab Total, but benefiting from Solomonic Travel.

Compare with Opening the Gift for magi which explicetly uses an Intellego Vim Lab Total, or for Learned Magicians where "the master’s Opening Total is his Succurro Magicam Lab Total".
Compare with Opening the Gift for Folk Witches, where it isn't a pure Folk Witch Magic Lab Total (meaning, for example, that a strong aura does nothing).

I guess the same applies to Familiars. Could a folk witch bind one without having Animal Ken?..

For a folk witch to bind a familiar, it must have an Animal Ken lab total. It doesn't have one unless it has the virtue, or is gifted (although since a gifted folk witch should end her apprenticeship with a score of 1, that's a mute distinction unless you're playing an apprentice or have a non-standard character).