The question of whether to route on the rail air bus to Paddington, or changing at Hayes/Reading or at Paddington is a vexed one.
There is no simple answer. It is one totally of personal preference. There is usually very little time difference in the Hayes and Reading change to the Paddington change, because the Hayes to Reading train stops at all stations. Coming off a long haul flight the easier the better IMO.
Yes, even at this late stage for next week there are advance tickets available and will be until midnight the day before travel. But that is a fixed time ticket for a specific train. Thus it is a gamble, whether your flight is on time.
Returns are cheaper than two singles, but not by a huge amount on the Exeter route. My advice would be to book a flexible single out from Heathrow and an advance on the way back. Check it out for your trains, but it is likely to be the cheapest combination.
Also on the way back look at trains from Exeter to London Waterloo on South West Trains. They are slower but massively cheaper on advance fares. To many people, myself included, far more comfortable trains and a much more scenic route. For Heathrow you can change at Basingstoke for the rai air bus, rather than going via London.
The even easier route is National Express bus- several departures a day Heathrow to Exeter.
The stop is right in Exeter City Centre.
Just get on at Heathrow and snooze your way. Most only have 1 or 2 stops.