I have an indoor trainer that can do watts, but I don’t have a power meter on my bike. The activity analysis says heart rate, cadence, and a1 are used, does this mean speed is not being used when estimating power?
Yes speed is never used for estimated power in our heart rate/cadence/grade/a1 model. That’s by choice, it might be totally different speed for the same effort on MTB than on road ride for example. The speed based estimate is a totally different approach, ideally you’d want to know the type of surface for that as well (for example Strava does). Both approaches are compared in Calculate cycling power without a power meter | AI Endurance
I asked one of the AIs and it said there is no field in fit files indicating if erg mode was enabled/not, and when I looked at the spec I didn’t see a field either.
I had had a thought that with speed, hr, cadence, gradient from an outdoor ride were combined with trustworthy ‘speed’ data from a trainer than those hard accelerations up small hills or soft-pedaling down longer descents could have better predicted power accuracy..
but that requires the trainer speed data to be pretty good and I don’t ever recall setting my wheel size and when you use erg mode it would feed bad data in so, 
Yeah I don’t think it would help the accuracy unfortunately…