I know its the only real flaw in the plan, but unfortunately precipitate planar breach doesn't allow you to choose the plane that it breaches, and there is no real way to make a living spell.
The living spell template may be odd, but it's description says nothing about it's exemption from other template rules. All you should need to create a living spell is a caster who knows the spell you want to apply the template to and a ritualist who knows the ritual needed to apply the template, (which could be one and the same person) if nothing else the wish major ritual will work, as per Chapter 11: Becoming a Monster from Savage Species.
Also, though your build is acceptable there's a much more direct way to handle it. Seeing as you have to waste a feat on Leadership already, you should just awaken the living spell rather than possess it; things you awaken tend to take a liking to you anyways and make good flavor for cohorts. A living spell gains the ooze type, which means it can be the target of an Awaken Ooze spell (DragMag 304 p38).
Also, for anyone more interested in living spells, and who knows, maybe it will help out with the build, there's an entire prestige class all about them, the Spell Sovereign (DragMag 357 p76-80).
