Easily.
At this level some spells take single targets out without even offering a save... and SR is no protection at all, anymore, unless it's ridiculously high.
Assay Spell Resistance plus some caster level bonuses makes that more like 10~20% failure chance. Add in Greater Spell Penetration and it's 0%.
Agreed, but this can be reduced if you force the wizard to drain his spell resources beforehand. And Assay Spell Resistance would be a good example of why decoys are useful to a villain. It's likely to get cast on round 1, and if the wizard didn't prepare any more and doesn't have a pearl of power, problem solved.
Yeah, using spells that target Reflex would be rather foolish.
Nothing a
quickened True Strike cannot handle, though.
Blatantness is often a factor in how useful a save will be. The wizard will see the robe, sandals, and martial arts, and will probably avoid area reflex spells entirely. This is why you should look to variants. Complete Mage has one to to swap Evasion for the ability to turn ranged touch spells back on the caster. That could be a nasty surprise.

Of course, with a monk, ranged touch also looks bad, but if the Wizard gets an idea like Thanee's...it's more likely to be used than Evasion at any rate.
I love 3E!

(NOT being sarcastic!)
He can also use Dimension Door or something similar, IIRC, so the Force Cage route isn't all that great a choice, anyways.
Yes, definitely don't use the Abundant Step frivolously. Save it for emergencies.
Yep, maybe not better than a Wizard with
Draconic Polymorph (those size and strength bonuses can get rather insane), but still ok.
But the bigger problem is, that grappling doesn't help, unless you also happen to have an
Antimagic Aura.
Actually, any time the enemy outnumbers you, grappling is an awful idea. Heck, even if he has a bunch of minions around, the PCs are all powerful and will look at him grappling and see "oh look, a guy with lots of dex bonuses to AC giving them up." The Rogue will see "free sneak attack!" So, they'll gang up on him regardless. Grappling is a bad idea for a solo boss or solo boss with weak allies. It can be done if you instead have 2-4 powerful bad guys.
Summoned monsters are distractions, they are not really able to take down serious threats.
But they can't hope to penetrate his DR by much, and he will be able to effortlessly tumble past them, so the won't even be much of a distraction. Also, by level 21, he'd better have some means of flight. There's just no excuse at that point.
Another good general trick would be going ethereal (class feature) +ghost touch property (enhanced on a gauntlet/shoe, weapon, weapon crystal, gauntlet of ghost fighting, ghost shroud, whatever the source).
If nothing else, the villain should have enough escape/survival tricks to inflict some quick pain and leave. If you actually manage to quivering palm a PC and then leave, it could create a plot hook where the party has to do a task for him in order for him to not kill the character. Perhaps not if revivification magic is cheap and plentiful, but it's a thought. Another possibility is to not announce to the players that you used quivering palm and instead leave his abrupt withdrawal a mystery. They find out why when they encounter him days or weeks later under either more favorable circumstances for him or what seem to be favorable circumstances for them as he opens up this "real" fight by willing the PC to die. Conveniently a free action, so he doesn't have to relent his attack as he does so.