Yes just buy your coach tickets at Heathrow. Fares vary a bit but upcoming departures tonight are between £8.50 and £10.50 per person. Create a free account beforehand at home, then buy on your phone with data or airport wifi when clear of immigration.
There should be a bus every 15 to 30 minutes, unless there has been an accident on the M4 or M25 when they may bunch up. Every bus has come from further afield (none starts at Heathrow) so they empty out on arrival. It would be unbelievably rare for them to be all full.
That way you avoid paying a £1.50 on line transaction charge, or £2 on the machines at Heathrow.
If there was a serious unscheduled issue on the trains most locals know other of numerous ways to get out of Heathrow by other means, so not many will be defaulting to National Express.
PS- Without knowing your travel date we don't know if there is engineering work on the tube but see this list for the next 6 months- https://content.tfl.gov.uk/planned-track-closures.pdf
It is a significantly shorter walk to the hotel from the Coach Station than from the tube station.