I wonder why it did not receive any response, are the questions unclear?
Without knowing a lot more about this bike and the key components (e.g., frame/fork quality, headset, bottom brackets, reliability of 2200-grade cranks & ring sizes/material, shifters), any suggestions to help band-aid the bike may only serve to help them get into more trouble later on. They really need to get themselves to a good bike shop somewhere where a no-kidding bicycle technician can put their eyes and hands on this tandem before they press on. That's the only sound advise I can offer.
1. How to build a sronger wheel for loaded touring, considering currente Deore hubs (135 mm specing and 36 holes) . This is the major concernAgain, this is probably enough "help" to really get them into trouble later on without knowing any more about their bike, their knowledge of bicycle mechanics, and if they will be heading off into remote areas where they will be "on their own" for days at a time.
An XT or XTR rear hub will have the same "guts" as the tandem-rated HF08 hubs from Shimano and be far more durable than the hub they now have. A 140mm Shimano tandem hub with left-hand threading for an Arai drum would be the best bet if they could get one (see below).
The original rims were far from ideal and chances are the current one they have may have a short life. What they "need" is first-quality, heavy duty rim and a very good wheel builder: Rhyno Lite, Velocity AeroHeat AT, etc... On the bright side, at least they chose a wheel with a somewhat common spoke count so finding replacements will be easier than finding any other drillings. If they don't have the skill and tools to replace broken spokes on the cassette-side of the bike they need to get that and a bunch of spare spokes before venturing further.
2. What to do about the brakes.
They needed an Arai drum brake but I believe that would require at least a 140mm wide, left-hand threaded Shimano tandem hub: not sure if 135 are still available and/or if you can shorten the axle and still get the Arai drum to fit. If the frame is steel, the rear frame can be cold-set to accommodate the 140mm hub without too much trouble: again, a bike shop or machine shop would be able to do this properly for very little expense. Getting their hands on an Arai and having someone who can install the thing on a frame that wasn't originally made for it is, again, something for a good bike tech or machinist to address.
The V-brakes are generally a bad choice for this type of touring and they'd do well to either carry several spare sets of pads (along with spare cables, spare chains, spare bottom brackets, and all of the tools needed to install these things.).
As for dealing with any challenging descents -- with or without the drag brake installed -- if they find themselves riding the brakes for more than a few hundred meters they'll need to stop the bike for rim / tire cooling and/or simply walk the bike down, lest they induce brake fade and invite a tire problem.
Speaking of tires, they should be running something a lot more durable than a 1.5" slick: it's no wonder they've had a bunch of flats. I would have opted for tubeless cross terrain tires and sealant backed up by tubes and spare tires.