Where do I find a single source of truth re: proper use and maintenance on M1 Batteries

1. You can leave it plugged in for weeks if you wish. The battery charging is managed by software but the power from the mains will power your device.
2. Depends on how much power the external device draws. If there is extra, your Mac will charge. Too little, and your Mac's battery will be used for the external device.
3. Let your Mac manage your battery. It's not ideal to drop to zero repeatedly.

10 percent per hour may be excessive but it really depends on use.

For Nickel-Cadmium batteries, your pre-conceived notions about how to use the battery would have been great. Todays computers use Lithium-Polymer batteries and are harmed by frequent and deep discharges, shortening overall battery life, NOT extending it.

That computer is a battery-CAPABLE device. It is not optimized as a battery-operated device. (It is NOT an iPhone.)

Your computer performs best when connected to AC power. It can use the full output of the Power Adapter AND when doing especially challenging work will also freely "borrow" power from the battery. In some cases, even with the power adapter connected, the charged state may even decline during stressful work.

When used only on battery, your computer has no extra cushion of power, and may perform more slowly. However, for ordinary non-stressful tasks this may not be objectionable (possibly not even noticeable.)

In general, you should ALWAYS connect AC power when it is possible to do so, and only run on batteries (which could be somewhat slower) when no AC sources are at hand. Your Mac will NEVER over-charge.

Your MacBook Pro uses ‘smart charging’ to charge in the optimum way, and only when necessary. Plugged in is Not necessarily actively charging. There is substantial hardware and software cooperating on battery and charging issues. Simply asserting a charging voltage against one of the interface(s) will NOT successfully charge your MacBook Pro.

I have a MBP/M1/Max (32GB/1TB) with a battery that is losing ~ 10% of battery storage every hour, which seems excessive, especially when you consider that published tech specs are indicating that users can expect 14 to 21 hours over an 8 hour period.

Those specs are done under ideal, laboratory conditions, running only 1 app and not necessarily on the model you have. For example, here are the specs & qualified notes on the current 14" and 16" Pro models. There are no separate specs or notes for the Max models even though they are shown on the same specs page as the Pro models:

Up to 18 hours Apple TV app movie playback

Up to 12 hours wireless web

Actual rating of 69.6 watt-hours (14-inch model) or 99.6 watt-hours (16-inch model). Testing conducted by Apple in November and December 2022 using preproduction 14-inch MacBook Pro systems with Apple M2 Pro, 12-core CPU, 19-core GPU, 16GB of RAM, and 1TB SSD; and preproduction 16-inch MacBook Pro systems with Apple M2 Pro, 12-core CPU, 19-core GPU, 16GB of RAM, and 1TB  SSD. The wireless web test measures battery life by wirelessly browsing 25 popular websites with display brightness set to 8 clicks from bottom. The Apple TV app movie playback test measures battery life by playing back HD 1080p content with display brightness set to 8 clicks from bottom. Battery life varies by use and configuration.

A Max model will inherently consume more power. If you are using WiFI and/or Bluetooth they continuously consume power; so does every bus-powered USB device you may have connected to your Mac; so does your display and the brighter you set it the more power it uses. Ditto for audio playback. There is no reason to believe that actual experience will match the tech specs for battery charge life.

"Mains" = wall (AC) power