In my view the runway width isn't an issue. It does slightly restrict the ILS to a worse that CAT I capability but with SEN's weather record it doesn't have such a big impact. Yes they have the odd day when it's closed and end up with a few diverting away, but every airport has that.
it's generally accepted upgrading to CAT II isn't worth the effort, it's either CAT I or CAT III and CAT III at SEN would be a non starter and for the significant investment it wouldn't have any gain, in my opinion.
The 737-800 'issue' if you can call it that. Well, who cares? I think an anchor tenant in easyJet is sufficient. I do think there is potential in the charter / Package holiday market at SEN but the Airbus can fill that gap happily.
As for the rest, I'd say it's likely to be more regional that works and with Stobart Air (aka FLyBE) with the E-jets and ATRs I think that operation would suit SEN well.
As for the current routes - it's way too early to tell how things are doing, but I think they will settle down, one or two may go, and a few new ones may appear until they get the mix right, but it's a great project and I think it will find its feet and do OK ... that;s if it isn't killed by the negativity and armchair CEOs on here in the meantime of course ...
