I've been fighting my perfectionism recently, and I have noticed that Divinity, Wizardry, and Psionics casters get spells in all 6 realms, allowing them to train said realms to 100, getting +10 resistance to all realms.

The Alchemy casters, for some reason, get no spells in the Mental realm (they get spells in the Fire, Water, Air, Earth, and Divine realms, however). Perhaps this was done for balance reasons, as for one, the Alchemy spellbook is the most balanced between offense and defense, and as for two, Alchemy casters can mix potions for money, something which no other caster can do.

Is it worth dipping into another class (Psionic, Monk, Bishop), to pick Mind Stab, to train the Mental realm?

The Psionic is the easiest class to gain a level up, out of the three. However, you would start with 0 Psionics skill, and 0 Mental realm skill. This would make casting even power level 1 Mind Stab a chore, with frequent fizzles and backfires.

Even if you get to the point where Psionics and Mental realm skills were good, and you could cast Mind Stab at power level 7 safely, is it all worth it?

Having an entire realm (Mental), devoted to just the Mind Stab spell seems kind of pointless, even if you somehow got your Mental realm skill to 100. Who is going to be casting Mind Stab middle game, or much less, late game?

The only benefit to this, is +10 Mental realm resistance, which can be overcome with Soul Shield.

if you have all the level 7 spells you need then yes its worth changing to Bishop in order to add extra ones... imo a swap to hybrid class like Ranger is better bang but thats not what you asked

I just realized, that I'm only "Human". If you want to cast Mind Stab at power level 7 safely, then you need 7 levels as a Psionics caster (Psionic, Monk, Bishop). Not likely, if you want to dip into Psionics/Mental realm for 1 level.

Of note, I believe that Psionics skill is locked, once you change back to your former Alchemy caster (Alchemist, Ranger, Ninja). However, Mental magic skill is still open, allowing you to cast Mind Stab power level 1 at enemies about 10,000 times, to max the Mental realm.

It is not worth dipping into a non-Bishop class, as doing so will cost you a caster level, and caster levels are very important for casters.

Dipping into Bishop doesn't cost that caster level, but once you do that, there's the question "why not make the character a full Bishop?"

Characters like Alchemists, Rangers, and Ninja have no Mental realm, unlike Divinity casters like the Priest, Lord, and Valkyrie, who might as well not have a Mental realm to begin with (the most honorable way I am training my Valkyrie in the Mental realm is casting ID Item on mixed potions, powders, and bombs. She is at 95 Mental realm skill, and my party has 1.5 M gold.

But why does my Ranger have to suffer -10 Mental resist, by not having a Mental realm to begin with? I guess that's just the way it is, Mental resistance can be made up for with Soul Shield.