"Are leather boots tough enough for the outdoors?"
Yes, Yes they are. a good one piece leather boot with wax for a good shine and water proofing is great.
one piece of leather means only one seem to come apart, not multiple seems all over the boot.
I've worn Vasque Sundowners for years,(they are goretex lined) never had a problem. I get at least a year and a half out of the Chinese ones.
I do have now and have had before the Italian made Sundowners, the mx2, and they are the best boot I've ever worn. I get at least 2 years out of them, and the pair I have now went out for new tread last year and I'm looking to get another 2 out of them.
Lowe, Asola, and Scarpa still make a full leather boot that will compare to any of the man made plastic or fabric or fake leatherette that the "modern" boots are made from most of the time now.
The Chinese Sundowners can be resoled too. the leathers not as thick as the MX2 but they are cheaper or were when they still made them in Italy.
I looked at the jump boots that you listed, and they are dam nice, but not hiking/backpacking boots. The Corceran jump boots I wore 25 years ago weren't that nice.
Just to sum up, DON'T discount leather, it's been tough enough for longer than then nylon will ever be.
Just to be clear I've never hiked a volcano but I have put more miles than I can count on a pair of boots in some rough country.
I do have no use for Keen boots, the sandals are great but the boots suck, I wouldn't recommend them.
good luck