WHBM
Regarding the Olympics, it's actually more like a 5 week period including the Paralympics but that's not really the point. The Olympic 'boost' is, I believe, seen more as the catalyst for airlines commencing services to/from SEN, particularly in the case of Continental operators. There would be no capital investment needed just to service the Games themselves, but it would give any new service a good start.
As far as the train service is concerned there is no prospect of SEN users not having seats to London and that prospect only exists for a very limited late afternoon period from London. They may encounter standing passengers en route, but I suspect that LTN users suffer likewise. The trains themselves are also likely to be of very similar standard to those serving STN and LTN. I don't see how the Euston to Birmingham International station comparison is valid, not least because you have to board another 'train' to BHX itself after your 70 minute journey from Euston. The Southend to Fenchurch Street line is complementary to that to Liverpool Street; there's no question of one or the orther being the "main railway" while that to Liverpool Street is marginally faster in fact.
SEN may not have a catchment to its East or South, but it has a very good one to its West and that from the North has much easier access nowadays with Chelmsford to SEN being a shorter drive than to STN. The total catchment is what counts surely. Its 'local' catchment (that giving a saving of 20 minutes in driving time from any other airport) is 608,000, while its equal distance catchment is a further 410,000. Its catchment for an equal train travel time from a mainline station is 1,583,000. Its Continental catchment for inbound traffic has to be added to this and that is virtually limitless and is an element often forgotten by critics of SEN.
The main reason that SEN lost its previous traffic levels of 700,000 p.a. is simply because the runway became too short for jet traffic and it was further shortened in 2003 when RESAs became mandatory. Any comparison with MSE once the SEN development plan is completed is way wide of the mark in my view.
One advantage that you don't mention is the reduced flight times from Western Europe which SEN will offer. An inbound from AMS or CGN is likely to arrive SEN much earlier than any departures from the same points to STN, LTN or LCY. Likewise, airport transit times will be speedier with 10 minutes looking possible from touchdown to station platform or car park with hand baggage, while the fastest I have done that at STN is 20 minutes. If an airline can offer its customers a quicker overall journey time into London by using SEN than does LTN or STN, then SEN comes into consideration. If it can also offer shorter flight times within that then it becomes an even more interesting proposition.
I'm not saying attracting airlines quickly will be easy, particularly in the current economic climate, but if the offering is right I believe they will succeed - not least because incoming airlines will be immune from the predations of RYR, which is not an insubstantial consideration in my view. Stobart's are the last people that one would label as being naive and I cannot recall any other UK airport where so much has been invested in such a short time with a runway extension, completely new terminal, new control tower and on-airport railway station. They at least must be convinced it will all produce substantial results.