Okay, I'm in Squidward's lair, and after spending all my gear on Furball and the Biskitts, I'm getting my wondefully sculpted ass handed to me by the Ubese. I don't have a single point in Stealth, not having anticipated having to solo my way through an entire enemy fortress, so do I have any options other than falling back on an hours-old save?

You don't need to try to solo the entire fortress. When you get the centre training room, I think thats what its called, just run to the left training room past all the enemies and activate the emergency door on the southern end of the room. That'll let your main character in.

I was able to fight my way through using an upgraded blaster and melee shields but still neded a good amount of health packs to clear the level.

I'm going to assume you took feats to set Mira up for ranged combat rather than melee. So try setting mines out of sight of the Ubese, use an alacrity stimulant to run faster than them and move forward until one sees and chases you. Lead him through a mine or two and you should be far enough away to gun him down before he gets close enough to melee. Repeat until you can reach the emergency exit and hand over killing the Ubese to the Exile.

I have to confess I've played this game so many times I always have an armour suit and a pair of upgraded blasters ready for Mira when she joins my party.

Lol yeah, feels a bit cheap though when I use the same pair for Atton as well,
takes a bit of timing to de-equip them so they'll be ready for the next guy.