Hi Nicola
I can't answer most of your questions but can talk about our experience.
My husband is very long term on Temodal, apart from some very small breaks since diagnosis in August 2006. The breaks were when there was tumour progression the first time and we tried etoposide for a while, but then went back on it on a daily dose quite soon after. We also had a small break when his blood counts dropped, only for two weeks though. We have never had a scan that the doctors would call clear in all this time. So you are in a different situation with your father. It is definitely possible to come off and go back on so that should not worry you. The length of time on it is also not a worry.
Good luck with your decision, in any case it is not a never again decision so going off it is not the end of temodal as an option.
Al the best
Irene