What is the best way to get into town from LHR? To, say, King's Cross if I choose the St. Pancras Hotel, as an example. Can I use the Oyster card for that or is LHR too far away?
Yes you can. You can use Oyster at every station on the
map here. (This is the one I linked to in your other thread - I'd suggest a Mod merge them are they are about the same trip.)
LHR-King's Cross is easy, straight on the Piccadilly Line. Takes an hour from T5/T4, slightly less from T2/3.
You could instead take the Heathrow Express to Paddington and Tube from there which is a bit quicker, but not worth the extra cost for the journey to St Pancras. Oyster is not valid on the Heathrow Express.