Lots of questions in the OP.
Starting at the top...
Prices vary massively for CU changes have seen quotes from £250 upto £500. You should charge what you know is suitable for your day rate if you know it's going to take a day. That being said I find CU changes are normally less than a day with testing after and the materials aren't the most expensive so it's normally well paid for it too stressful a job.
As a personal choice I like the high integrity 2+5+5 boards, gives scope to go none RCD which can be a necessity at times (garage supplies (swa) and the like).
As for testing, you'll get quicker with practise. Yes remove all the loads etc including neons but don't chase the perfect result, just record what you find. If it's below acceptable then I spend a little time double checking I haven't missed something, but not too long that it becomes a new job in itself. I see that you say you are not allowed to produce EICRs, though any qualified electrician can do that, even with out 2395, although the bodies frown upon it. I'm with NaPiT and they insist on both testing courses but I'd happily produce an EICR without a professional body. Only need them for part P really. Just make sure you have professional indemnity insurance.
I don't test before changing a CU, would take just as long as looking to do a quick rectification after the change. Though I do have a quick visual inspection on all the usual items. If they have a major issue then we can deal with that after. That's where some trust comes in.