I spent most of the winter on a treadmill because it was the only way I could get much “uphill” in my training. I live near a hilly area but I can’t access the hills during my weekday training window. With so much training on a treadmill, I had a lot of time to think about those exact questions and here’s my thoughts.
It seems to be an okay way to maintain a certain volume and even build volume. You certainly miss out on the effects of the uneven surface of the trail but I compensated as best I could by doing specific hip and glute work, lateral hops, Icky Shuffle, etc. I would also do “side shuffles” on the treadmill every few minutes. It’s not perfect but it’s better than nothing.
For elevation, going “uphill” on a treadmill is great but you miss the downhill. I did some backward running/walking to compensate for that, even though it was not anywhere near as much as I would have gotten outdoors. Not perfect but better than nothing. For logging elevation, I keep an Excel sheet because I’m on a free TP account and I kept a separate column for treadmill elevation separate from my outdoor elevation because most of it on the treadmill was “uphill” and not up/down so you get more “ascent” during the same workout time.
For my TSS, I used a fudge factor of 3 TSS/1k ft “climbed”. The normal fudge factor is 10/1k ft climbed/descended but the treadmill is mostly climbing so I missed out on the eccentric part of the run/walk so I guessed about 5 TSS/1k “climbed”. Then, because the treadmill is moving under my feet I adjusted downward a little bit more and came up with 3. It’s not scientific but it seems to work and roughly match up with an outdoor workout. Depending on how much sideways and backward running you do, maybe 2 or 4 would work but 3 worked for me.
The mental part is the hardest but there’s always podcasts!