I was wondering if casting non effective spells in combat could raise train skills. Insanity / itching skin against savant guards / savant troopers.

Casting the spells you mentioned will indeed count for practice purposes. An offensive spell doesn't actually have to work to count as practice; it just needs to target something. Even a spell that fizzles or backfires will count; in fact, such a spell will count *more* than if it were successful.

I also note that Terror, I believe, is not completely useless on enemies immune to insanity, as it has a very small chance of inflecting unconsciousness. I haven't seen it inflect that status when I used it (though I have seen it inflected by an enemy's spell against me).

There are a few things to watch out for:
* Heal Wounds and Heal All need to restore some HP to count.
* Stamina and Rest All count\ only if cast during battle for some reason, but will count during fake combat (that is, starting combat when no enemies are nearby, then casting the spell).
* Spells like Bless, Chameleon, and Ring of Fire only count if used in real combat; fake combat won't work here. (Note that using Summon Elemental during fake combat *does* count.)
* Some non-combat spells have what I call a "practice timer". If you cast one of these spells, it will set a timer, and until it runs out, no further casts of that spell (by any party member) will count as practice. Note that the game does not save the time remaining, so if you are playing on a modern computer, save/reload can be used to purposefully reset the timer for more practice. (Note that this isn't a good idea on what I call "period computers" (that is, computers common around the time the game was released); the load times are just too long.) Long-duration spells like Armorplate have practice timers, but so do some other spells like Hypnotic Lure and the Portal spells. Be aware that spells that fizzle or backfire will still set the timer.
* From what I can tell, the amount of SP spent on the spell *does* count for purposes of raising Spellbook and Realm skills, but not Powercast.

Casting one of the "traveling spells" aka missile shield, armour plate, enhanted blade, x-ray etc while they are active
will only grant the chance of a skill increase if you cast them at one or more power lvl(s) higher than the already active power lvl.

Actually, that's not how it works.

These spells actually act on what I call a "practice timer", as I mentioned above. Whether the spell is active doesn't matter, nor does it matter whether or not the spell is used at a higher power level than before. All that matters is whether the spell has been recently cast; if it has (regardless of success), and the game hasn't been loaded since then, the spell will not count for practice.

As I mentioned above, you can cheat the system (even on Iron man mode) by saving and reloading after each cast to keep getting practice. Note that this is, I believe, more cumbersome on Iron Man (because I believe you have to exit the game and restart after each casting), and is likely not worth it on a period computer (as it takes too long to load your save), but on a modern computer without Iron Man, it is actually rather fast.