Welcome Bleadon. It's always good to have a new local poster on the BRS thread.
The western walkway was built as general permitted development under the Town and Country Planning Order 1995 which means that formal planning consent was not necessary, although North Somerset councillors tried to say it was but eventually realised the error of their ways.
This means the walkway cannot be treated as an extension of the terminal in the sense that it will increase floor space so the airport can only use it as an alternative way of getting passengers to the stands on the western apron. That's the reason why the airport is limited in its options for furnishing the walkway, though I believe there may be some provision for people with limited mobility.
I suspect the airport will use the new facility 'imaginatively' and it ought at least to remove some of those snaking queues in the main departure lounge, though wait for the inevitable moans about having to walk over 400 metres to reach an aircraft. It should certainly be more user-friendly than the current metal-wrapped partly open corridors on the eastern apron.
You are right about the problem of overcrowding being in some ways a nice one to have (a bit like a football club with a ground not big enough to accommodate all its fans), though I still maintain that there are too many retail outlets taking space originally occupied by public seating. The snag may be that if the overcrowding is not addressed some people may decide to go elsewhere.
According to the BEP this week, the major expansion planning application will be heard by North Somerset councillors on 28 April, not that I expect their decision to be the final one. Appeals, legal challenges or even a public enquiry are likely from one 'side' or the other depending on the councillors' decision.
A change of national government might also have a bearing on whether the expansion goes ahead.