Question 1.
pretty good chance of beating ST it's not an assurance.
If the target (let's say an heavy damage dealer) saves and close range (or he's a ranged damage dealer, an archer?)... the caster risks seriously to die (a Wiz/Sor 20th even with a nice Con 16 has more or less 90hp).
The problem. Look at the damage output. To begin with, the 'tank' isn't going to survive that either. To follow up, its also why a good number of casters open/precede fights with buffs like Invisibility, Blur, Mirror Image, Blink and stuff like that. Most of those last long enough that if you expect a fight in the near future putting it up wouldn't be unwise.
This tends to happen as soon as the slots can be spared. Conjurors can also get Abrupt Jaunt and fast summons to use a straight no-sell ability or an alternative target, respectively.
Question 2.
Forcecage... yes nice... to run away: it's not exactly a fight winning spell... especially granted that a damn disintegrate is sufficient to break it.
If you use the closed wall version, you cannot do anything to the target.
If you use the barred version, he has plenty ways to slip out.
Yes, a single disintegrate destroys it....how many monsters have it, that you would want to encase them? Brute types are removed by the closed cage, you deal with them at your leisure later, and against caster types the action they spent disintegrating the cage or teleporting out is an action they aren't using to tear you a new one.
As far as I remember, one single balance breaking (especially in 1v1 situations) is Death by Thorns (BoVD) granted that even on save, incapacitate the target.
But indeed it's a Corrupt Spell... I'd dare to say it'll be really rare to see.
Try better spells like Solid Fog(no teleportation? Too bad, enjoy being sidelined from the fight for the next two rounds while they pound you from outside the fog with AoEs), Holy Word/Blasphemy. Or Shivering Touch(save half? who cares, halving the dex damage still takes out 80% of monsters). Thats taking a direct conflict approach, going around the problem is available via invisibility, illusions, teleportation, divination, Calling spells etc.
More in general... I'd say that if "monsters have pretty big chance of failing saves vs optimized DCs on their weakest saves", the caster needs to be quite specialized.
Granted the enormous options in D&D, every player with a bare minumum of experience can create a build + gear and an "action sequence" granting almost 100% success vs. a choosen enemy. But it's completely situational.
Situational...no either.
All you
really need is to put your highest stat in your casting. If you're an arcane caster, this means you can afford to ditch Str and either Cha or Int completely, so getting a starting 18 isn't difficult. Then you put your stat increases from level into it. Then you throw in a +X Enhancement to the same. Then you grab Spell Focus and Greater Spell Focus(which is not a problem, because most spell schools allow you to target a variety of saves, and the bonus applies generally). You already have a high enough DC to get 70-80% failure chance if you target the monster's weakest save(which you should be managing on everything but outsiders). Thats basic spellcasting optimization, you hadn't even touched hijinks.
So the caster's "situational" win is the "situation" where they have an applicable spell from their favorite school. This is easily 100% of situations, unless you're playing an immunizable type of caster like an Enchanter(and even then you can dominate NPCs and critters to deal with the problems you can't) and Necromancers(see above, but replace with animate zombies).