I've taken it upon myself to examine the guidelines used for each spell and actually determine to which guideline the build section refers. As a boring example, there are 4 separate CrIg10 guidelines. For the most part this is an uneventful exercise. However I've run into a few (more like ~50) problem spells I'd like to throw out there and see what you all think. I'll keep this first post to the few main book spells as I'm sure you all will point me to the errata page I've missed or something equally embarrassing.
Sight of the True Form's guideline has a variable InIM base. This looks like two separate spells crammed into one. First is "see through [physical] mundane masks and disguises" (my brackets) at base 10. The other is a general guideline "can see through the effects of other spells that are equal to or lower than the level of this spell." This is the only spell that has such a "variable" guideline, and while I understand there is value in legacy spells, this spell seems weird to me. Prehaps someone can come up with a suitably worded general guideline to accomodate this spell. Or is anybody for two spells here?
Exchange of the Two Minds is ReMe base 20. The only ReMe20 guideline I see is "give a person one complex command." I'm not sure how "complex command" is the same as switching minds? ReMe30 is "completely control" a mind. This spell more than completely controls a mind, it controls two minds and is base 20 to boot. I don't see how this is base 20.
Wielding The Invisible Sling has a base of ReTe4 but I don't see a base 4 in the ArM5 book. HoH:S has a level 5 guideline "Hurl a stone projectile with enough force to do +5 damage" which is exactly what WTIS does. Is there an errata I missed?
Trackless Step also has a guideline of ReTe4. "Control dirt in an unnatural way" is level 3, with a magnitude for stone would be 4, but one tends not to leave footprints in stone. I'm thinking this should have a guideline of 3. I'm not sure why this is 4.
The mental durance to check all of these must have been just overwhelming to any editor. If multiple editors, coordination and agreement only add to the task.
Also, many are "legacy" effects - classics that were just too good or too popular or too "canon" (to the genre, if not the game) to get rid of, but really can't be explained via the Guidelines.
IMO, your are asking the most relevant questions about the rules. There is many discrepancies in the central rules of Ars Magica. You do well in asking about them.
Maybe each gaming group deal with it in it's own way. My own is the base my reasoning for assign a level for a new spell not only on the RAW guideline, but also with doing some deductions from many sources: the ArsM5 spell exemple being one such source.
I think you will find this page ( villamanlia.wikidot.com/inductiveht ) will help you answer your questions. I'm not the writter of that page, but I found it to represents one interesting way to answer such question about the rules.
Exemple:
For the game balance question (exchanging minds as BASE level 20 versus control one mind BASE level 30), you have the choice. If you think such spells to be too powerfull, you can declare this spell to be a "Legacy" spell, and it's not possible (or difficult) for a maga to change the Range, target, etc to make for a new spell. Or (like I would tend to do) you go for the simple solution: the ReMe20 guideline about "switching minds" was omitted (in the ReMe guideline list), but it exist nonetheless. There is many cases like this. I think each has it's own way of dealing with it. There is no "good" answer, just prefered ones.
Of course, as Cuchulainshound seems to point out, the discrepancy in the rules does not stop me from thinking Ars Magica is the best RPG around. Nor does it mean the editor are doing a bad job.
Nor does the Central Rule (p. 111) should stop the people from this forum to find some problems in the rule, discuss about them, and form solutions.
It turns out I started this project after reading that page.
I can totally see the unwritten guideline for switching minds. It's easy to get tunnel vision when wading through spell guidelines and not see the "easy" fix. Thanks.