Shimano Road 10 speed "mullet"
 

Without getting into the minutia of the gearing I want on a build I'm thinking of (and thus avoiding the always helpful comments and advice that I really don't need such low gearing):

What wide range RD's are compatible with Shimano 10 speed road shifters? I'm not sure of the model of my 2X10 speed shifters, they're the early version with the external cabling (not under the bar tape). I've had them in the parts box for quite a while but they're relatively low usage and in perfect shape.

I know that 8 and 9 speed MTB RD's are compatible as I've built a couple of 10 speed road bikes with MTB RDs of the 8 and 9 speed type. That will work fine if I decide to go 2X (which is feasible since I have both right and left shifters!)

If I want to go 1X, I'd like to use a cassette that will take a 46t large sprocket. I see that there is a currently model Deore RD-M5120-SGS that looks like it will go to 46t on a 1X drivetrain, but as I currently understand it, the 10 speed MTB RDs are not compatible with 10 speed road shifters. FWIW I also have a set of Cane Creek brake levers that will pull the canti brakes on this build, so I have the left lever for this option as well.

I think the Microshift Advent wide range 9 and 10 speed RDs are apparently not compatible with anything but their own Advent MTB shifters? The XLE 10 speed long cage RDs say compatible with Shimano MTB shifters, so I assume this means they have the same compatibility as Shimano 10 speed MTB derailleurs - not compatible with road shifters.

I'd appreciate any corrections to my understanding, information and advice, thanks.

Unless you are completely committed to using those older Shimano STIs, here is something "completely different" to consider. Gevenalle shifters are available in both 10-speed road and MTB format for 1X, 2X or 3X cranks. For that matter they are also available in 11-speed and you can use an 11-speed MTB cassette on a 8/9/10-speed freehub. These have external cables but so do your STIs.

Bicycle Cassettes & Drivetrains - Google Sheets Check this chart. Main thing is to look for any derailleur with the correct "Pull Ratio" for Shimano 10 sp shifters which is 1.7 for any older external cable types such as yours. Also lists a few other details such as max/min cog, capacity, etc. but only the 1.7 ratio will work. Also lists some Microshift derailleurs that may work but not the XLE as you suspected.

Get a 9 speed Shimano MTB rear derailleur to go with your 10 speed Shimano STIs.

That derailleur won’t clear the large cassette the OP wants to use. Typically the largest cog a 9-speed MTB rear der will clear is 36t.