Avent pumps are, imo, rubbish. Hire the symphony, you won't believe the difference. I used a medela swing and t'was ace.
Don't feel trapped, just take it one month/week/day/feed at a time. Just like the early days. Then you aren't trapped. You are giving the next feed/x amounts of feeds because it suits you to do so, and you can revisit that decision at any point.
Get the formula prescribed and in the house, and then you have it for when you decide you want it. Absolutely no reason to feel bad about that, you have had a great breastfeeding career, and may go on to do mweeks/months/years, but of course you wish to start planning a life and building all these demands on you into it.
Get your nights planned and begin the great express in preperation for them. Start early and you will be really pleased to see how the milk starts stacking up. Give some to a friend or family member too, have heard to many stories of power cuts, broken freezers and lost milk 
Also, you can do both. Happily you aren't having to be dairyfree [it really really really tested me, I so missed nice tea] so you can breastfeed the feeds you want, and introduce formula for the feeds you want. You are an established breastfeeder and have total control over it now 
I absolutely had nights away, and expressed to bank up milk for those nights. After about 8 months, I introduced the formula and hadn't the pressure of needing to express in advance. DD1 was starting school and having time to express was a stress.
One tip, don't expect heaps of magic from the dietician. Like all things in life, they can be...er...variable
Have you see a paed or is the gp handling this? Ideally a paed would be overseeing the process, but that said if you are all happy with the diagnosis and management then why worry.
It is rather isolating having any problem at all, I reckon. Praise the Lord for mn!