why havent you planted it outside in a protected location?
Is it not on a hardy rootstock?
I bet your zone 8a is more like 8b, that should be plenty easy for a citrumelo no?
It is on Poncirus but I simply don't have enough space. My grandma allowed me to plant two rows of hardy citrus in her garden and that is where I planted one of the Nr. 82 that I grafted from my tree shwon above.
In the last twelve years, we have not seen anything colder than -10/-12 C which would be 8a. Many winters have in fact been 8b or even 9a, so you are right.
However, our winters are long and damp and we can have prolonged freezes where temps stay below zero for days and sometimes weeks. A hard freeze is usually not followed by positive temperatures the next day. And that is what kills citrus here.
Moreover, while some citrus may be hardy here, most of them fail to ripen their fruits in time before the first severe freezes. Our growing season is simply too short. My in-ground plants usually don't flower before May.