Apologies for the delay in replying, I finally travelled on the bus replacement service today, for the first time.
The answer is that they are indeed using the middle doors too. Which I used when alighting. As someone who's not accustomed to using them it's quite striking how useful they are. Saved me a bit of time anyway.
All stops on the circular route have one or two members of staff permanently assisting passengers. So that would probably deter jumping on via the middle doors. Although, it's not likely much of an issue in this case, as the service from what I've seen on a Saturday at least appears a bit underused. And no-one seems too terribly fussed about scrutinising tickets.
I can understand people who may not be able to walk very far or very quickly or who may have bags of shopping would find this bus replacement very useful. But generally, an unencumbered person walking a brisk pace would be able to beat it almost all of the time. This being due to the numerous traffic lights and the road system in Liverpool city centre. This possibly explains why only something like 1% (at a rough guess) of rail passengers using Moorfields station seem to be using it. Well, that and the fact that Moorfields isn't in such a bad location for the shops etc.. anyway. Travelling on the upper deck almost feels like taking a ride on a tour bus, particularly, when you're right next to an actual tour bus.
On the rail-replacement bus service, looking down Water Street towards the River Mersey.
As for Liverpool Central station itself, the Northern Line platforms still look unchanged, except that there are now metal barriers erected around the platfrom egdes. The Wirral Line platform however has now had the 1970s style plastic fittings removed, exposing the concrete lining behind them. So, they seem to be getting on with that quite well so far.