Hmmm. I must say, you seem to have a few options.

1) Why go through all the trouble of making your SS mtb geared? Just tour singlespeed style. That means carrying less weight, moving faster, and maybe gearing down just a little. You could always carry a lockring tool and a spare cog for any long climbs. There's plenty of folks (myself included) that take weekender tours on fixed gear bikes, it isn't as hard as it sounds only having one gear.... okay..... it is hard, and you have to like a little pain. It's good for you.

2) And a bike that is a little too big isn't necessarily a deal-breaker for touring. I ride a 58cm road/race bike, but a 61cm touring bike. Good to stretch out, and sit up a little. So maybe you could borrow the Panasonic with just a little tweaking for fit.

Sounds like either of these options would be safer and less worrisome than taking the Alan.