Yes, I'm aware that the firmware file they provide is not the same thing as the boot rom, that's why I wrote them in separate sections.
The switchmode power supplies are a big part of why the NLA1 gets noise complaints, but if that's how you like your synths...

The linear supplies of the L1/2/3 are, IMO, a big plus from that perspective.
I don't know how big or drastic your fluctuations are, but I've never seen any power related issues with my NL3. Did you check the capacitors in your power supply? Maybe they were just going bad. Linear regulators are perfectly capable of tracking fluctuations (as good as switchmode, and better in some cases), and headroom is simply a matter of current capacity, not topology.
Where did you find the information that version 0.2 of the boot rom has diagnostics, or how to use them? I'd be curious about this myself, as I want to keep mine running as long as possible, too.
Do you have the pinout and chip type of the ROM chip? Is it a standard JDEC SPI flash chip or is it older than that and needs a special reader / programmer?