Eldritch Theurge Glaivelock, viable?

[PId6]

Actually, it is. If you at any moment not qualify for a prestiege class you have levels in you lose all class features in said prestiege class. So using this for early entry, you would only have the PrC features in your sanctum...until you get 2nd level spells.

Being "able to cast X level spells" just requires that you're theoretically able to cast that level of spells at your current level. Otherwise you stop qualifying for all such PrCs whenever you run out of spell slots, become prevented from taking actions by some means, or fall asleep.