Moro orange graft died

Hello,

I have a 3-4 year old moro blood orange, I've had it for 3 seasons. It's about 3ft. tall (pruined for size) with about a 1.5" diameter trunk. All summer did fine. I bring it in when it gets below 45 and keep it inside all winter. The past two winters, all was fine. This year when I brought it in, it began dropping leaves. Then one branch turned black, then another, etc. until the entire top of the tree was dead. I assumed the roots failed somehow, but left the "dead" tree in the pot and continued to water just in case. Today I noticed two green buds below the level of the graft line, and a very clear demarcation between the black dead grafted part, and the healthy green rootstock.

Questions:

Any thoughts as to what killed the scion? Leaf drop, then limbs turned black slowly. No bugs. No bound roots (trimmed them up at the start of the year and repotted). No overwatering.

Will the roots survive when there hasn't been leaves on the plant in weeks?

What rootstock is typical for this tree (I think it's a semi-dwarf, MIL gave it to me) and is it worth continuing to grow if it is alive?

It's a shame, it was very healthy and growing like crazy all summer.

Thanks.

Maybe the sudden change from outdoors to indoors? Was it in a draft, or too near a heater or something?

Rootstock may be a green shrub-thing, but almost never has a tasty fruit. Rootstocks are chosen for disease resistance, efficiency in taking in water and nutrients from the soil, and the dwarfing effect. Most end up being aggressive, thorny things that nobody would want except perhaps as a barrier to keep people off the property. (If it would grow outside. It would not in your area)

Sounds pretty scrubby, any value as a bonsai? I feel bad throwing out my favorite plant if it's not dead, and it has such a nice big trunk. The house was warmer than outside, probably drier, but all the other plants came in at the same time (includng meyer lemon), and this is the only one that had an issue. There was even new growth from one branch after a few weeks when I brought it in, even that branch began to turn black as the new growth continued to grow. The entire process took about 3 months.

There is a possibility it was left out for a soft frost, but I thought moro was cold tolerant. Maybe the roots more than the scion. I'm not sure if I ever expected to get fruit cause our season is short, but my lemon did give me one small lemon this year.

There are some citrus that are more interesting than others. If yours turns into something you want to work with, then maybe it is something like Flying Dragon or one of the others.
Flying Dragon is one that is noted for the thorns. They are so long and such an organized arrangement that an imaginative person can see a flying dragon in a branch. Google some images!

That's pretty neat looking, all curly and spikey! Would make a mean looking bonsai. I guess we'll see what kind of monster my tree will turn into.

Also, is it possible to graft off my lemon to this rootstock? Sorry for all the questions, I don't have too much experience in citrus as I live in the tundra, and this is all fascinating to me.

It's interesting that flying dragon is deciduous. How does this effect non-deciduous grafts?

Since the rootstock is not expected to grow leaves, it seems to be just fine for an evergreen scion to be grown on a root that would be deciduous if it were allowed to have leaves.

I am not sure which citrus are compatible. Would be worth trying!

Here is more info about Flying Dragon and related rootstock.
http://edis.ifas.ufl.edu/hs221

Thank you!