Are there any good advice posts hidden in the 369 pages of commentary?
I don't believe there's a overall "guide" per-say. There's a FAQ in the OP that should help with most things you're likely to get stuck on, courtesy of Shade.
Probably upping your combat stats first and foremost, just to avoid losing any of the game over battles. I'm sure there's a few posts detailing which stats are best for combat if you're struggling, but I've never had an issue on the normal difficulty.
things worth/not worth buying,
Adventure books are useful for upping stats.
Skill books for skills.
Green Tea gives you a bit of energy, so I'll buy that early on.
Coffee lets you stay up later, which is pointless until you have a larger amount of energy, but eventually you might find it more useful than green tea. I think black tea was a mix of green tea and coffee but less effective at what either of the other two do.
I buy the pillow, bedding, weights, and shoes as soon as they're available at the store in the town. The bedding lets you recover more energy when you sleep and the weights and shoes improve your stat gains from training. It takes a few weeks for some of those to appear IIRC. I don't think anything else is particularly useful for actually beating the main story line.
what some of the less-clear bits of the game do, point totals to aim for before trying things
Too vague for me to try and answer.
if it's possible to do anything particularly lewd at night before hitting the 1 AM mark, etc? As usual, the search function is giving me mixed results.
Get her stats high enough that she isn't pissed when you wake her up at night, satisfy her, repeat a few times. Eventually you'll be able to ask for sex anytime in the afternoon, which gives you as much time as you should need.
Edit:
Don´t waste money on Dumbbells, Training Shoes etc, your Sister will gift them to you randomly if you keep her happy. Not sure about the Comfy Pillow or Duvet, never got those gifted.
Didn't know that ^. You tend to get them early on as gifts? I buy them as soon as possible for the stats haha.
Edit: I'd also recommend a separate save every Friday (weekly) in case you hit a bad end. Being able to redo a week should save you from most if not all bad ends unless you've really neglected your stats. Also, don't rush the dialogue. You get hints about getting another party member and a lot of people seem to miss those hints and miss acquiring said party member.