Jo - please click on my name to see my profile, it has similarities to your Dad.
When I was first diagnosed I made it clear that my priority was quality of life rather than quantity of life. Rightly or wrongly I therefore turned down the opportunity of going on Stampede because to me it felt like we would be firing off all our bullets in one go. Again who knows whether I was right or wrong but I'll live (or die) with that decision.
Given my priorities my Onco advised the HT route. I'll continue on that route until there's virtually nothing left to try apart from Chemo. Thus it will be an easy decision to make when that time comes.
My quality of life has been really good for the last two years, hopefully it can be the same for your Dad on Enzo. There have been a couple of downsides with Enzo which you can see if you read previous posts I've made.
It costs the Cancer Drugs Fund about £90 per day for the Enzo I swallow. I reckon its a good bet your Dad's Onco would only be suggesting he does Enzo now and keep Chemo for later because he/she thinks it's the best option for your Dad's quality of life.
Regarding Enzo v Abby, we do only get one shot at one of these. From the little I understand most Onco's seem to prefer Enzo to Abby.
I hope this helps.