Well, as far as I vaguely remember, they closed Corby on the grounds that Corby-ites could jump in a car or on a bus to go 'down the road' to Kettering. TBH, looking back, I'm surprised they were allowed to get away with closing Corby when they did, but I suppose at the time it was a rather self-contained town centred on its steelworks, ie it didn't generate much traffic, holidays to Strathclyde apart. Kettering is a much bigger place, and they would not have been able to close Kettering and Wellingborough.
Mkt Harboro might have been dodgy under this scenario, I agree. They could have kept Leicester - Harboro - Nothampton open as a single-track branch, and closed Harobor - Kettering complete. But I think there would have been an outcry from Luton, Bedford, Wellinboro etc at losing an outlet to the north, so I think it would have survived, but been singled (with loops, of course) Bedford - Wigston.
Well, it's all conjecture, of course, but cash-strapped or not - Euston would have had trouble coping with an extra two expresses per hour by 1980 or so, I should think. And pressure would have come on to put these trains back on the Midland. It could have been done incrementally. Would not need all re-doubling, at least not all at once.
Actually, I did originally begin writing about this aspect [ie closure of St Pancras] but then deleted it, as I thought it would muddy the issue. But yes, it, ie a diversion of MML to Rugby (or Nuneaton, which was also supposed to have been considered) and singling north of Bedford probably would have added to the pressure on St Pancras.
However, although there were all sorts of plans mooted, I suspect that even at top managerial level there were thoughts that singling north of Bedford would be a "rationalisation" too far - especially after electrification on the WCML brought on such immeidate and significant growth. It would be really interesting to know how seriously these rationalisation proposals were taken. I mean, closing the GC was one thing - between Aylesbury and Rugby there were more cows than people - and it should have been obvious to everyone concerned that running three semi-fasts a day between Nottingham and Marylebone after 1960 was a joke, and obviously not a serious attempt in any way to attract passengers. So as freight traffic waned, the writing was on the wall for the GC. But you could not possibly get away with running down the Midland in the same way.