- I know this is not exactly what you asked for, but this issue is filed instead that might address your use case: https://github.com/GrapheneOS/os-issue-tracker/issues/28
Basically, it will allow you to have your long password/passphrase as the primary unlock method (needed to unencrypt the phone after a reboot) and have a combination of PIN + Fingerprint as a secondary unlock method.
This will allow you to be able to enter a 4-6 digit PIN and then scan your fingerprint in daily usage, which while not as quick as just the fingerprint mitigates your concern of someone being able to force your finger into the sensor, and you won't need lockdown mode, since a PIN will also be required by default with no fallback.
There's an open issue on the tracker for this: https://github.com/GrapheneOS/os-issue-tracker/issues/1007
Indeed, this has been added on the latest release, and more improvements to the feature are coming in the future.
This strikes me as the first thing that's potentially out of scope. The screenshot function just takes a full screen screenshot, and editing/cropping it is delegated to a compatible editing app (AOSP Gallery by default, but can be changed).
This, as you said, has been discussed a lot of times, and the power button gesture will not be used for the flashlight. It's possible that it will be used for other things in the future, which are much more within GrapheneOS' overall scope. One possible application of it I could think is double-pressing the power button reboots the phone to put your data back at rest.
Aside from that, while this may not address exactly what you're asking, it seems that AOSP is planning to make it so you can add a flashlight shortcut right on the lock screen (on the bottom right/left) once configured by the user, so when that makes it into AOSP and naturally then makes it to GrapheneOS, you'll still have to turn on the screen, but it'll make it much faster, which is hopefully an acceptable compromise.
- Unlikely that AOSP default behavior on this will be adjusted downstream.
I hope this helps! Keep in mind that I'm only a community moderator, and not on the GrapheneOS development team, so this is just my take on things.