If by "realism" - you mean "sounds like an acoustic kit" (edrums *are* "real" - just not acoustic ;) )
...then yes, a 502 is a good choice - multi-layered sampled expensive Yammies and Zildjans in there (not models). All down to budget - a 502 module in the UK is typically about £150. And...
Depends on your needs and wants and what issue you have with the DM10. Why "upgrade"?
The 500 was replaced by the 502 and I'd choose that over the 500 - it's not much more. (or go 700, or 900 or etc). Note - you'd have to get a Yammy hat controller. The Alesis pads should be OK.
Mic up his acoustic? Yamaha EAD10.
As for electronic - usual advice - get thee to a music store and try stuff out. Go hit the Roland you mention. Go smack a Yamaha DTX (try both the silicone and mesh etc. pad options). Listen to the sounds (Roland modelled, Yamaha multi-layered samples)...
[opinion as requested]
On those two? DTX. No argument. Many more features, layered sampled sounds (not models), 3-zone snare (head + cross-stick + rimshot), 3-zone ride, 3-zone crash, silicon snare (no meshy hotspots and much quieter), up to 12 pads natively supported, dedicated app if that's...
[scary opinions incoming. Only the brave read further]
Opinion as requested:
TD1 is dead end.
Alesis is fragile.
Both have evil proprietary cable snakes.
Get 2nd user higher-level kit for more for your money.
"Best" is the one YOU/HE/THEY try, like the sound of, like the feel of, has the functions you/they want (download and read the manuals), and fits your budget. (and doesn't have an evil proprietary cable snake, obs).
You ask on a public forum, you'll often just get a list of what people own as...
Same pad badged by at least 4 people. ddrum Nio, Gear4Music DD90, dbDrums NPad, HXW/Avatar PD705 etc etc
Plenty of reviews on the 'Tube.
As to what's good for you depends on what you want to use it for :)
DM5 is mono inputs. So - you can try - like you did with the ride - take two cables off a dual-piezo pad. If the pad only has a single socket then use a stereo --> mono + mono y-cable to separate it out.
DM-20? Doesn't exist? There's a DM-10 made by Alesis.
As for Roland models - (a) download and read the manuals - all online and free. (b) there's charts on Wiki of kits/ modules/ pads...
1. basically yes. You'll need to think about cymbals + hat. Could use these instead of mesh
2. You could but keeping them there would be a pain. This is an option...