I've been out of touch for a few days so many thanks for doing this sleuth work once again.
Would you mind clarifying..... are you saying it may be hard to sort out which William is ours- John and Sarah Stevenson baptized their child in December of 1772 which matches best with the death record that shows the William who died in Tiverton was born in 1771. Our other option is the William Stevenson/ Ann Jerram birth in May of 1769 which now seems less likely because as you say the christening would have been at age 3 which doesn't seem all that likely. It seems to me, and I'll run this by Prof is we go with John and Sarah as parents. This might also account for the choice of John as a name for the first child of William and Elizabeth Rogers.
I'll check Family Search to see if the records were 'submitted' on that William Stevenson. It seems that his marriage (in 1797) may be in error and also this death (in 1799). Again you weren't able to find either. I didn't come across of death for a William Stevenson in Loughborough in 1811 so maybe that's the correct death record for this other William.
Thanks so much!