Stem- We fit quite a few of these types of stem adaptors each year. They work well if the rider isn't a hammer head that is going for big air all the time. Actually the majority of the brands we sell have off road limitations in their documents. The stem adaptor has no bar clamp, that's for the stem to determine. (When I clicked on the link a stem adaptor and not a stem was pulled up). So you need to know the Jones bar's clamp diameter and the adaptor's "quill" diameter (1.125" per link) and the reach and rise of the stem you will seek.
MtB as touring bike- Sure, many have done this. There are some considerations that often generate opinion
One is that most MtB frames have a longer top tube length than a road bike of the same rider "size". (Remember the hand position is behind the stem/bar clamp on most all MtB bars and ahead of the clamp for drop bars). So for some a drop bar brake hood reach is rather longer than they had with flat bars on the converted bike. A shorter stem can mitigate this somewhat.
As this bike was made for a rigid and non suspension fork is a good thing. Front racks and fenders fit so much better on most rigid forks.
Any handling claims for this bike after a drop bar conversion I will take with some "salt". Handling is a very complex thing WRT bike design and the rider's contribution/perception is a large part of any opinion. Added to the crazy weight distribution of some touring bikes and without a lot of added data most all claims are specific to only that rider on that set up with that load.
The Rivendell- Not enough stand over clearance? Or too much reach? I would think these bike have a greater value on the used market as a complete bike with OEM spec, not a mish mask cobbled to sell or a frame/fork sale only. Andy