I've been using a 1l Nalgene bottle to carry water for at least 15 years and it still doesn't impart any taste I can detect to the contents. Of course I might have picked up BPA poisoning, but that's a different issue.
To answer my own question, the Melmerby shop is still open and in pretty good condition. Both the downstairs rooms are clean enough and have good stoves, and the upstairs rooms are quite habitable, though a bit dusty. I would have given the one we were sleeping in a sweep out, if there had been a brush available. There's a stream pretty close at hand; the water is a bit peaty, but perfectly drinkable.
Many manufacturer's make walking shorts out of the same material that they make their trousers.
Usually a fairly tough polyamide/nylon.
So if you're going to rip your shorts you're just as likely to rip long trousers.
Companies like Berghaus, TNF and Rohan usually have shorts versions of their trekking trousers.
Berghaus for instance have the Pitzal and Ortlier shorts and TNF the Paramount and Meridian shorts, which are just shortened versions of the long trousers with the same name. And considerably cheaper than the Haglofs shorts you mentioned.
The Sprayway shorts I mentioned are in this style. They fit well, have the features I want (basically they're not too long, have a zip fly, belt loops and they're not too long) and show no signs of wearing out after at least five years in use.