I'm going to answer in reverse-order.
3. I really like the Killer-Gnome/Shadowcraft Mage builds. Be an Illusionist with Evocation and Conjuration as barred schools, then take
Spell Reprieve to get Greater Teleport. Maybe even
Arcane Transfiguration if I can spare the feats. That doesn't always work though, so I usually wind up with Abjuration barred instead of Conjuration. Then I take Spell Reprieve to get
Arcane Turmoil.
Don't forget your early-entry shenanigans. Heighten/Earth Spell with Metamagic reducers. Precocious Apprentice. Etc.
2. Depends what spells you use. If you like Touch Spells, Arcane Reach is invaluable. Especially if you like Chain Spell. Anyone who Blasts even part of the time and has even one ally needs Mastery of Shaping, and probably Mastery of Elements. (Fire Mage vs Red Dragon? Well now you're an Ice Mage too. And an Acid Mage, and...) I usually don't bother with Spell Power. If you're starving for Caster Levels by 20th-level, something has gone horribly wrong.
1. Spell-Like Ability depends on what spell you want to take with it. It costs one fifth-level slot, plus one slot of whatever level the spell is. So taking it for a fifth-level spell is pointless, and fourth or lower is worse than useless. Shapechange is a ninth-level spell, so it's almost definitely worth it. As an Archmage, you already have two Spell Focus feats. Is one Transmutation? Then try out
Metamagic School Focus with Extend at the very least. Maybe throw on two more +1 Metamagics, alongside whatever +0s you have. Remember that SLAs don't have Verbal or Somatic Components, so don't bother with Silent or Still Spell.
What level are you starting at? A permanent Shapechange item costs... 459,000 gp. Okay, so not WBL until at least level 19.
Master Specialist... Depends on your School, and how lenient your DM is. For example, I once convinced a DM that the capstone for Illusionists was underpowered, so he let me use it at-will instead of 3/day. For a Transmuter, I probably wouldn't take more than four levels for the Minor Esoterica.
If you want to cheese yourself into semi-permanent Shapechange, all you really need is
Incantatrix 3 (be sure to get the Player's Guide to Faerun version, NOT Magic of Faerun),
Persistent Spell and the ability to reliably hit a DC 63 Spellcraft check. Remember that you can combine Persist and Extend for a 48-hour duration (which also bumps the DC up to 66).
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Although, if shapeshifting shenanigans are all you really want, might I recommend
the Master of Many Forms Bible instead? I'd recommend going in as a
Wildshape Ranger instead of a Druid.