Mileage accuracy on trainer

As others have stated... pay zero attention to distance and speed on the trainer.

Power

Heart Rate

RPE

I greatly disagree with ignoring speed/distance on the trainer.

If you are using a KK or Fluid2, the speed and distance on the trainer are very reliable metrics of how hard you are working (speed) as well as distance (training volume), and on those two trainers, is very reproducible. 

The issues brought up above are more for comparing your trainer speed/miles to other people metrics or outdoor miles, which isn't a great comparison for the reasons above. 

However, for the most crucial part of training, which is comparing YOUR results to YOURSELF over time, so you can see if you're improving and how much you're improving, tracking your speed/distance on the trainer is extremely helpful and very precise for good fluid trainers like KK or Fluid2. 

Remind us again how far and fast you travel on the trainer.

As said, indoor training numbers are good for comparing to other indoor workouts.

As for comparing bike workouts, the critiques you make could be applied to riding outdoors, different routes, in different temps, terrain, etc., Even a powermeter doesn't account for all the variables (although it does make it a much closer comparison.)

You will get the highest precision in comparing bike fitness progress by comparing your TT efforts on the trainer, and get valuable data about RPE and even HR or power if so inclined for race day. 

Or just use the speed/distance you get inside but don't try to correlate it with outdoors (as was already said).  Using 'virtual' power is essentially the same thing as using speed on the trainer.  Faster=more power, slower=less.  Not sure why people who think virtual power is awesome would tell people to ignore speed, as long as you do the same things in trying to keep resistance and tire pressure equal from ride to ride.

More generally, for the OP, mileage based training is only helpful if your terrain/conditions are somewhat similar across all your rides.  You could have two VERY different 50 mile rides, for example, with very different training stresses.  You wouldn't want to call them equal when thinking about your training.  So don't worry about the distance.  Focus on time & effort (which you can track with speed on the trainer, if you like).  If you are comfortable enough that your riding terrain is relatively similar, then mileage will work well enough.

Yes, tension between the roller and the tire does matter. Can't recall the specific formula but a google search should give you the procedure. Something about spinning it up to a certain speed then timing how long it takes for the wheel to stop spinning on it's own. Based on those figures you can calibrate the tension to be closer to what the road actually is. Most likely depends on brand and type of trainer as well, probably different numbers for different trainers. 

Virtual Power = function(Speed)

If Virtual Power "means something", then so does speed.  And, therefore, so does distance.  Just understand what it means.  And what it doesn't. 

What you said earlier about comparing speed between trainer rides is correct, and in that reason virtual power is useful, as it gives a more precise measurement.  How much of an improvement is a 1mph increase in speed on the trainer?  What pace would create a Zone 2 effort?  Pure speed doesn't quantify these as well as mapping to a power curve of a trainer to calculate a virtual power. 

You've posted why distance isn't important, so I'm not sure why you made the bolded statement.

I agree there is some added value in understanding how much harder you have to work to go 1mph faster on a given trainer (e.g., might not be the same as 1mph faster on another trainer or going from 17-18mph might not be the same as going from 20-21mph).  Virtual power does that math for you so you don't have to worry about it.  But saying that speed doesn't mean anything makes no sense since it is the measurement of rider output used to create a virtual power number.  Knowing that I rode last week at 17mph and this week at 18mph does tell me something (I rode harder this week). 

I never made any kind of blanket statement that distance wasn't important.  I posted earlier why training by distance on the bike might not make a lot of sense given that it may not consistently 'map' to your combined effort and time.  On an individual trainer, it actually does (if it didn't, virtual power wouldn't be possible).  Like I said, just understand what each metric means and what it doesn't and they can each be useful in their own way.