1. Not as far as I know, but if you are on facebook there are countless groups you can join, some quite good at that kind of thing.
2. red diesel is still available, and at your own declaration at present - although HMRC has had its wrists slapped by the EU and is maybe going to change it if we dont leave soon!! basically, the present system means you declare how much diesel is used for propulsion, and how much for living (heat/light/electricity generation etc)...propulsion is charged at full duty price - heating type is currently around 70-90p a litre. if you have a dedicated tank for a stove - this will be 100% domestic.
3. bowthrusters are fitted regardless of boat length - it is down to the commissioning owner - and there are no hard and fast rules.....many of us have up to 70 foot boats without, and dont struggle. If the boat is 60 foot exactly - you can probably squeeze through the whole system (a couple of locks you will have to reverse through).
4. if your asthma is serious - diesel stove may be better than solid fuel because of the dust. Gas is an expensive way to heat a boat in Winter yes. Spray foam is best. There are coal boats across the system which usually supply diesel and gas too - some do other things like pumpouts - but not all.
5 There are modern apps and software, but the canals dont change much. the books you used to use will probably still be fairly relevant although many named pubs may have changed/closed.
6. Toilets - pick and choose - empty cassettes several times a week, or pumpout once a month. Smelly(MR)l be along to tell you the cassette is the best thing on the planet - although his present boat is a pumpout. Composting - some say too much trouble - getting too in touch with your waste - however , seem to be getting more popular.
Merchant is still around.