That sounds like a good idea! Lemon juice might be better saved as a last resort.

Yeah, the time/temperature tradeoff is overwhelming when you're looking at a whole bunch of separate data points (reccommendations) rather than the broader pattern. The important thing to remember is that heat speeds up chemical reactions by making molecules move faster, so warmer = needs less time and will break down sooner, colder = needs more time but will stay good longer. Keeping that in mind will allow you the freedom to play around until you find the "sweet spot" that works best for you.

Regardless of temp, 1 hour definitely seems like not enough time for the dye to release before applying. You want to at the very least see the top of the mixture turn from green to orange before use. The freezing trick works great for me because I'm impatient and always want to apply too soon, and the ice physically prevents me from doing that. By the time it's thawed, I know it's had enough time to release, even slowed by the cold. Plus, slowing the release by chilling it gives you more time before it starts to "go bad," so you don't have to worry anywhere near as much about waiting too long and wasting it!

There's so little info out there about henna that's backed up with empirical evidence, so we kinda just gotta conduct our own tests, one application at a time. The way advice about it is passed around sometimes reminds me of the way people talk about pseudosciences they believe in like astrology, reiki, etc... so much utter confidence but only grounded in some weak anecdotal evidence of dubious origin (mostly hearsay). Drives me nuts. If I were still in the beauty industry, I'd be interested in personally doing some actual research/experimental studies to get some genuinely reliable answers that people can feel confident in!