UK trains: I bought a return ticket from London Euston to Coventry. Can I board at different station?

The answer as with everything to do with UK train tickets is not clean cut

But a simple answer is most tickets, including ones marked Anytime, off peak, super offpeak do allow for a break of journey. So in this case you could join the train at Watford, informing the gate staff that you broke your journey.

However if you hold an Advance Ticket you must travel at the time specified, and from the stated original and to the destination stations. In all likelihood you would not be allowed though the gateline at Watford .

It is possible that should you manage to get through the gateline at Watford (for example by buying a ticket to the next station Watford junction to Watford high street is £3.20!) that you might be able to join your original train (as long as it was the one originally specified on the ticket. You would be at risk however if the ticket inspector is on the ball, of it being spotted that you were not on board from the original station, assuming that the inspector conduct a check between London and Watford (in my experience this is unlikely!). The ticket inspector would be within their rights to demand full payment for a new ticket at the most expensive rate for the journey.

Yes, this is explicitly allowed by the National Rail Conditions of Travel:

16.3. Generally, you may start, or break and resume, a journey (in either direction in the case of a return Ticket) at any intermediate station, as long as the Ticket you hold is valid for the trains you want to use. However, this may not be the case with some through services that take an indirect route. You may also end your journey (in either direction in the case of a return Ticket) before the destination shown on the Ticket.

The exception about "through services that take an indirect route" is not relevant for your ticket.

(This exception is aimed at services that loop back to their starting point. For example, some Southeastern High Speed services go on a clockwise loop around Kent: London St. Pancras - Gravesend - Chatham - Ramsgate - Dover - Ashford - London St. Pancras. Due to the rule that a direct train is always a permitted route, one may use a "Anytime Plus High Speed" ticket from Gravesend to London St. Pancras to go all the way around on this train. But you can't use that ticket to start your journey in Dover instead).