Regarding US/EU 3790>SG/MY 3790:
Yes it should be able to successfully convert to SG/MY because of the matching PCB P/Ns, however you'll then still need to be able to source (or maybe adapt) some required files for it, voices for example. Many files will be SG/MY specific and might not be so easy to find there days. Incompatible US/EU specific files onboard will just soft-brick it on first power up, most likely that's why it wouldn't boot on your last conversion attempt.

Regarding 3790>3490:
Many of the *.ASR and ASR-related voice control files are indeed map-dependent however some are not. i.e. Some will need updating with the map but others stay the same.

If your device has Lifetime Map entitlement of NTU Europe it'll be easier after conversion because GarminExpress will continue to supply map updates based on it's Unit ID along with appropriate associated files based on its HWID (if available). If it's an earlier 3970, even if it's LM, it may have been originally factory programmed with NT (non-unicode) EU maps and unfortunately many of the associated files meant for 3490 (NTU use) are not suitable for the NT maps (because 3490 was only ever supplied with NTU). Many early 37xx didn't have an LM entitlement either, only a "One-Off" update was guaranteed. I think yours might be a later issue because of it's 8GB flash size and higher UID, so it's likely not just LM but using NTU maps anyway.

So in short, if your 3790 already uses LM NTU it will be far less problematic to get correct files after conversion to 3490 than if it has an earlier NT entitlement - whether that's LM or One-Off in NT.

Although some 'old hands' reading this thread might scoff at your efforts to revive your crippled device and my attempts to help you thru it, i can fully understand your attachment to it and the 'emotion-based' desire to keep it running. I've got more ancient Garmin devices, some well over 20 years old in fact, which i keep going for the same reason even though their monetary value is a mere few dollars nowadays. Please press on and good luck.