Answers.

1. No. I think it's a case of both nodes having to be able to communicate to set the fee-rate. If one channel partner is unreachable, there is no way that the other partner can be guarenteed to close the channel within the accepted parameters of both nodes fee policies.
2. Thunderhub, doesn't know for sure what the state of channels are (reachable/unreachable) at the time before you submit a request to either unilaterally or cooperatively close.

If you try to doit when a channel is unreachable, it'll assume it is and let you attempt to propse a fee-rate and then attempt to execute it. But you'll receive an error like:

UnexpectedCloseChannelErrorr. unable to gracefully close channel while peer is offline (try force closing it instead): channel link not found

i.e, it just won't allow it. You'd have to force close and accept the unknown fee-rate estimation.