Hi Weaver, and welcome!
On your questions:
1. If you feel stable in your routine and are hungry to move on, then by all means, test the next step. If it is too much, you can back off as needed. No harm done. Keep in mind that a "stable routine" can be a bit illusive, meaning, practices may have a delayed reaction in effects. You will usually know within a few weeks or months what the short to intermediate term effects are from adding new practices, but some cycles of purification are longer. It is like driving a car through a changing landscape. Sometimes the road is straight and smooth. Other times it will be winding and bumpy. We adjust our speed accordingly, yes? Same in yoga. The longer you are doing this, the better you will get at self-pacing.
2. While traditionalist teachings put a lot of weight on keeping straight posture without back support in sitting practices, I think it is highly overrated -- the equivalent of a puritan fixation or something. Certainly not worth sacrificing comfort, which diverts the attention from our practices to the discomfort. Who needs that? The AYP approach is comfort first and posture second. It works quite well that way.
3. We all have what we have time-wise, and we will be gaining every day we are in practices. I think the "here and now" of yoga is most important. It is an improvement in the quality of life we can be cultivating and enjoying today. Looks like you have at least 20 years to work on it. That is a lot of "todays."
The maturing of sexuality (less reproductive urgency) is actually an advantage. It makes the tantric aspects of yoga more manageable. The most effective tantra is pretty laid back. And don't worry, you will not "run out" of the energy.
Finally, whatever we accomplish here spiritually we take with us. Nothing is ever lost. All the more reason to get on with it. See this lesson:
http://www.aypsite.com/plus/74.html Bottom line, whether we have one year to live or 50 years, yoga is worth it -- for today, and for ever more.
Will you finish in this lifetime? Not sure. It depends on how much housecleaning is necessary. Everyone is different in that. One thing is for sure -- a couple of decades is a lot of housecleaning, especially now that we are getting more efficient practice-wise, and certainly even more efficient in the years to come as applied yoga science takes off.
Wishing you the best on your path.
The guru is in you.