Have not fully read the rest of the thread (quickly skimmed -- I'm on a time budget, forgive me) -- but --

An external SSD is essentially a conglomerate of three separate parts. The SSD proper (always just a standard drive), the enclosure in which it is placed (with a USB/FireWire/eSATA host controller PCB), and the power brick or wall wart. Some (typically cheaper) enclosures substitute a second USB lead for the power supply.

Luke, have you opened your external SSD and verified that it is indeed the SSD itself which is at fault, and not the enclosure's controller board?

You can very easily get (probably easier than I can, since you're basically right there) what eBay calls a "USB IDE SATA" adapter (I call it a drive tester) which adapts IDE and SATA disks to USB. Those require a separate SATA-compatible power supply, or a brick terminating in a Molex connector, plus a Molex-to-SATA adapter. You may be able to find SATA-only versions of that which incorporate a power supply -- but those rely on a USB3 port to do their work, and probably won't work on a USB2 port. (There's a chance, with SSDs -- spinup on a notebook form factor platter drive is usually five watts, fully twice what a standard USB2 port can provide.) I don't know whether your Mac has USB3 or not -- as a general rule, I don't touch stuff from That Fruit Company that isn't beige or tan. (I have a IIgs.)

If it *is* the enclosure, the SSD will at least read, when paired with an adapter as described. I don't know about MacOS, but in Linux, if there's any trouble mounting a drive, the drive will be mounted as read-only (or not mounted at all). If it mounts 'ro' it's probably fine, just needs whatever Mac's equivalent of fsck is. Even if it doesn't mount -- run fsck or its equivalent -- a truly corrupt F/S will not mount.

Obviously, if it's the enclosure, don't go for the cheapest replacement you can buy. I've seen enclosures from eBay where if you don't hold the cable exactly right it disconnects the drive. Those are crap and you shouldn't buy one.

Phew, that was at least a textual footwall. Sorry about that, folks!