Elijah Smith:
1) Are extended retreats necessary to move through the dark night to first path?
Varies individually, from what I can tell. It takes effort either way. It's possible to dally away a retreat, apparently, and it is possible to buckle down and have stream entry happen off-retreat, as I can assert. Having good friends helps either way, and getting guidance, either from all the resources available or again from good friends or teachers, helps either way.
It's not just practice time (though that is a major bit). Check out the 5 faculties, for example, for a more balanced picture.
2) Several sources seem to describe the A/P and dark night as being quite extreme in terms of altered sense experiences ...
Others claim that the dark night can resemble more traditional psychological conditions ...
Are both approaches valid?
Again, the way this "map territory" presents is highly individual. The descriptions in MCTB are of the more extreme sort. Daniel is a medical doctor, so he listed all the symptoms, risks, and side-effects he knew of. He even makes that comparison.
Explore, observe, attentively investigate, (insert favorite term here) your own living, in-your-face experience (of this territory, for example). The maps help a lot, but don't get trapped into tracing out the journey on paper only, so to speak. While the paper is undeniably part of your living experience, it is only a tiny portion of it.
Cheers,
Florian