For the 17.31 nonstop to Paris I suggest being in St Pancras International station, in the queue, by 16.00.
That will be a very busy rush hour train full of people going home to Paris after a day at work.
The latest you should be leaving on the Underground from Heathrow is about 15.00 because the tube takes just about an hour.
So you want to plan to be arriving at the car place an hour before that. Or earlier depending on your tolerance for things happening and how important or expensive the trip to Paris is.
The fact is that things sometimes happen. That's true on the Underground, it is true on overground trains, and it is true on the roads.
I tend to have only a very small tolerance so I tend to pad things out to account for problems.
It is substantially under an hour - depending on traffic - on the M4 past Heathrow to continue into London and return the car at St Pancras. Check which car you are renting and where the return is. I haven't checked but I think I've heard that Hertz is inside the congestion charge area and Avis and Europcar outside. But check.
Or leave plenty of time for returning at the airport and getting in.
As far as using the Heathrow Express, you then have the most expensive train in the UK per mile (maybe nearly all of Europe), and then a change at Paddington which isn't the easiest station to navigate onto either a taxi (more money), the tube (but you could have done that at Heathrow and saved all that money for a very similar time), or the number 205 bus (very slow, much slower than the tube). All to perhaps save 10 minutes or so (taxi or tube).
Unless the tube has sprung a leak (see what I did there?) I'd drive in or take the tube from Heathrow, leaving plenty of time for either.