Hope you have both had a good night sleep and Joe keeps improving so well.
High dose chemo sounds very scary and daunting - does he get radiotherapy as well? The positive thing is that so far his cancer seems to have really responded to the conventional chemo, so you would expect that there is a very very good chance for the high dose chemo to eradicate it.
It is going to be a long slog though. You know that mumsnet will be there and we will all try to help you, but of course support in real life is the most important.
Not sure if this is an option, but would you have some close family/friends (even if not necessarily nearby) where you dd could go for a break, so you both can concentrate on Joe? It could be packaged as a special treat for you dd as well. Maybe not possible for the full period but even if it is for the initial period, so you can settle in a routine.
I hope that your husband will be able to get compassionate leave. Where I used to work we would probably have granted compassionate leave in such circumstances, although possibly not for the full two months (or part would have been unpaid, I would expect), may be worth exploring though.
I think there is no way round it, it is going to be tough. I suppose he is going to be in two months isolation - is that correct? If so, is there anything you want/need for him, or anything he himself would like, to keep himself busy in what is ultimately going to be a boring, confined space? Please let us know, I am sure we can organise it amongst ourselves and get it to you.
Thinking of you, as always, and don't forget, Joe has already come so so far. He is a true star!