That, I'm afraid, is the logic of the conspiracy theorist. The estimates were reached at by standard scientific means and were universally accepted at the time. It is quite possible that the figures were an overestimate (especially as many of the 'reserves' were extremely deep) and they certainly don't count as 'reserves' now, but that's not really the point. Coal was not running out in the 1980s; the industry was shut down for other reasons.
Basically yes; and if 1980's assessments are not proof enough, recent assessments of the West Fife coalfield (where open cast mining still takes place despite the flooding of Longannet) for example suggest that there significantly
more coal that they had assumed even 10-15 years ago.
Coal 'rests' while it waits for technology to catch up (clean coal and carbon capture) to make it economically viable in the UK once again.
Worth pointing out that had '1984' not happened, coal would have been a casualty of the turn of the decade 'environment' boom at any rate (and closures probably met with more public sympathy), however the rate of decline would have been far less severe.
