A45 : There is also Illusion Mastery, though I don't know if that provides spontaneous use? It would qualify you for Signature Spell though, assuming you're high enough of level (can't remember those prereq's). Also, there's always the Dragonblood Pool. Uh...Illusionist 3/KotW 1/Ultimate Magus 10/Shadowcraft Mage 5/Shadow Adept 1 maybe? Would be casting as an Illusionist 19/Knight of the Weave 8, with a CL of 23/12 and plenty of extra slots to power even more metamagic on Shadow Illusions.
KotW is actually explicitly against Shadow Magic! I'm going Wizard 3/Spelltheif 1/KotW 1/Ultimate Magus 10/Exalted Arcanist 5. It gives me 9th level spells from wizard, 6th level spells from KotW, and a caster level of 33 for both classes. EA is there so I can go nuts with Holy Word.
A feat that would let me qualify would be best, as I can just retrain it later.
Mystra is also explicitly for subsuming the Shadow Weave
Tis what the Twice Betrayer is based on, actually. If you can use one of the methods mentioned to be considered spontaneous, you can get in with Versatile Spellcaster, using two 2nd level slots to make a 3rd level slot. I do like the Holy Word idea though, even if it's not shadow miracles
The Shadow Adept dip isn't needed, and could be replaced with Mindbender or something. SCM was for versatility and raw power, not to mention if you felt like blowing shit up from time to time
I know it wasn't the main focus of the TS TO operation (which has evolved into a Ritual of Alignment native outsider...), but a total of 360% effective reality (20th level slot, +20% SCM 5, +20% feat) spells PAIN with a nice high cap or no cap blast spell. Especially with CL 40 (or higher with gear).