Few points of order on this.
Firstly, the arrival of the 12 plate E400s were not rushed. Generally most of them received vinyls and were repainted in plenty of time. I understand that there were a handful of engineering issues with some of them too which had to be sorted before leaving Hengrove.
12 plate E400s - generally speaking they aren't bad. From a driving perspective, they accelerate well and are comfortable to drive. However the brakes aren't the best and on some, in particular the ex Weston ones, the brakes snatch. There are two annoying problems I find with them - one is the change pot - most of us have change dispensers, so having to count out change each time you get on a different bus can be quite annoying, especially when you're late. This wouldn't be such an issue if we only had to carry around £1 and 50p coins for simple fares. The other problem is the n/s wing mirror - the legacy of these buses for me will be the pain in the neck that I'll forever have. From a passenger perspective, I have very little issues with them. They're generally comfortable, however I agree with your points about USB charging points and the heating - we can't control the heating and its rather annoying. I'll come onto USB points later. The worst thing about all of these E400s is generally the non-existent suspension - going over any speed hump or hitting a pothole is particularly bad, especially on the 14 plates. Each one of these buses have their own problems - however 33661 and 33664 have generally been well behaved. 33754 has problems with starting, hence why its off the road. 33658 and 33660 are generally brilliant buses, but just breakdown quite a bit. 33662, 33666, 33667 and 33754 are the worst for snatchy brakes. 33659 used to have very bad problems accelerating on hills, but has thankfully been fixed. And finally 33665 is generally a good bus. All of them rattle, some more than others, but like I say all are different. They are well suited to 376, but in my opinion would be better suited to 172/3/4 (age related).
14 plate E400s - the first one of these I drove was 33835, which I was impressed with. Generally the 14 plate E400s have a vastly better braking system - but that's all they have going for them. 33838 doesn't have any change pot (a nightmare), 33840 rumbles when above 40mph and 33836 was littered with problems. All of them have significantly worse suspension and the acceleration is awful. Generally they can't decide what gear they want to stay in and they are rather annoying to me. The headlights also have a filter which lowers them - so you can't see as far along the road as you can with a 12 plate.
MMCs - by far the best type of decker I've driven. They could do with a bit more room in the cab and the brakes are rather "Streetdecky", but generally they're pretty good. Not the best with acceleration, but still cope with the hills particularly well. The only real issue I have with them is once again the n/s wing mirror - which is positioned in an even worse place than the older Enviros. These, as in my previous opinions, would have been the best bus to buy for 376 in 2015 over the Streetdeck and we wouldn't have suffered all the problems that we've had previously.
So yes I agree with some of your points, but generally speaking the E400 deckers are a far better choice over the Streetdeck and some are even better than B9s. I'm generally pleased with the approach of the fleet - having the same type of decker is a good idea and using 54/55 plate B7RLEs (of which I'm hopeful it'll be 54 plate, since they accelerate better) on 126 is a good idea. If First withdraw all the B7RLEs, they'll be short of vehicles, which even before this crisis they couldn't afford to bring in new stock to replace. So keeping some of the best singles in service on routes which don't get affected by any emission zones are ideal in my opinion.
As much as it will surprise some, I can't wait to get rid of the B9s. Although most have had to have new gearboxes (I believe 37605 is about to get another one only after a few months of getting a new one), they have been pretty reliable. The big issue that I find is they aren't designed for hills - 37605 is particularly bad and will get down to 15mph before deciding it wants to go up the hill. The 09 plates are generally better and are more comfortable.
With the B7RLEs, all of the Wells drivers will be mostly disappointed at them leaving. They're an excellent bus in so many ways - I've been privileged enough in the last few weeks (before going on furlough) to drive most of them on 172/3/4, some for my whole duty. They will be deeply missed, but they do unfortunately have several problems creeping up - the only one that hasn't had any issues is 69443 and, obviously, 66720.
If we can get a fleet of E400MMCs, I expect most of us will enjoy them, especially since the 14 plates aren't anywhere as good.
When it comes to USB charging points, I fully understand your frustration, but generally speaking USB charging points, as others have said, aren't a necessity. However saying that, 376 carries a lot of commuters and many that need to charge their phones. I've been under the view that if all of Wells was run by Stagecoach, it would all come under the Stagecoach Gold brand, so would receive such luxuries as comfortable seating and USB points. I think that all our routes also need to have next stop announcements - as much as it annoys drivers, its very useful to passengers. I don't think unfortunately we'll see anything like this anytime soon under the current situation.