Citrus trees maybe dying after front?
 

I live in southern Utah, zone 7 or 8.

Last spring I bought four dwarf citrus trees (Valencia orange, bearrs lime,
eureka lemon, ruby red grapefruit) and kept them in their pots. They got
hit by some frost outside a couple of times, and their leaves got brown and
started to fall off. I brought the four trees inside.

The orange tree recently got leaves again. The other three seem to be
greenish on the root stock, but I can't see green under the bark on the
upper part. The branches are brittle, but there seems to be a little
softness under some of them.

Little sprouts started coming out on the root stock...I figured those were
suckers, and I removed them. Was that the right thing to do?

If a tree grew well the previous year, then does a healthy root stock mean a
healthy tree, or is it possible for the upper part to die while the bottom
stays fresh?

Thanks for any info, advice, etc. you can give.
--S.