1. ...Don't even consider running editing on a ramdrive - it's simply too much grief if you lose your work. There's a good chance there will be enough background I/O anyway that your main drive wouldn't spin down... aaaand they don't even use that much power.
Hmm. I think you may find with most RAMdrives nowadays that stability is probably not an issue - unless the OS is unstable, of course.
A good example would be using a dynamic RamDisk with (say) ImDisk Toolkit. I've been trialling that for a few months now, and it has performed superbly. It's a simple set-up-and-forget service, and quite trouble-free.
2. ...Screen is definitely the biggest power drain, especially when you're not running anything compute-intensive
Yes. I've always assumed this to be likely, though I didn't collect any data to back it up.
3. ...the hardware change that would be most likely to get you more battery life would be, well, the battery - especially if the computer is more than a couple of years old. Batteries wear out over time.
Yes. A failing battery is a PITA. A new/second and/or larger capacity laptop battery is a worthwhile investment and reduces such annoyances. Extends the happy/useful working life of the laptop and the user experience too.
4....A SSD might be a good option - you might not get much extra battery life out of it, but it's a big quality of life thing. Less noise, faster program startup, much reduced vibration (nice if you use the laptop in your lap). And do be careful to check reviews first: in laptop situations (especially when it's been possible to spin down the disk), some brands have actually ended up being more power-hungry than traditional disks.
Yes, I would intuitively expect shorter start-up times for SSDs, though never having used an SSD I wouldn't know for sure. An improved user experience would probably be the telling factor there.
However, I do wonder what hard disk drives
you have had that reduced your QOL by vibrating or being too noisy.
I mean, I have never
experienced such problems on a laptop hard drive (even failing drives), over the years. It's generally the fan(s)
that will tend to vibrate and/or become too noisy, because the heat-exchanger is clogged-up by bits of airborne fluff and flakes of dead skin to the extent that the waste temperature rises through lack of adequate cooling, so more power is fed to the fans to improve the cooling. Taking apart and thoroughly cleaning
the heat-exchanger grill, the fan enclosure (and the fan blades too), and a shot of CRC on the fan spindle usually transforms it into a cool and as-good-as-new state, believe me.
Oh, and for a cool-running, long-lived and more trouble-free laptop, DO NOT
rest the laptop on fabric or on your lap!
Place it on a smooth surface (e.g., a tray) which is resting on your lap or on some fabric, and place a 1½-inch long pencil eraser in the centre under the back edge of the laptop (try it out). There's minimal restriction to the airflow that way (and see above re fluff, etc.). Be kind to your laptop.