I really don't want to use those other MIDI channels (1-4) that I typically use for other instruments.

I'm with you there.  Between my guitar synth, large keyboard collection, and rack & floor FX, there's not much left for soft synths.  That's why I decided long ago that - if I'm going to run a complex system - I'd better invest in hardware MIDI routers, multiple MIDI Ports, merge and thru capabilities, event processors, and plan my system down to the component level.  It also meant becoming intimately familiar with software features, like MIDI channelizing plugs, the Event Inspector, and 'redirect' features like those in Bounce To Clip(s).

it still didn't answer why I'm only able to use one instance.

Just an opinion, but multiple instances for a drum kit is a bit like setting up multiple drum kits onstage, and then running around to play them all in real-time.  But sometimes, that's what it takes.
 
So, here's how I set that up.  I insert (in this case) four instances of Dimension Pro.  I get rid of all of the extraneous tracks [track folders, and all but one of the paired MIDI tracks].  That leaves five tracks: One 'master' MIDI track, and four 'orphaned' Dimension Pro audio tracks.  (I move the master MIDI to Track 1 for flowcharting clarity.)  It's helpful to relabel the tracks, but unnecessary for this exercise.  Using the Program Browser, load up each instance with the specific whole patches (presets; .prog files) that we've discussed above.
 
I don't know your MIDI input device, but set that accordingly.  The Drum Map is fed from the Output of the 'master' MIDI track.  In the Drum Map manager, you no longer have to be concerned with MIDI channels.  Dimension Pro's normal state is Omni mode.  The major area of interest is the Out Port setting:
 
Dimension Pro -1 (= Kicks & Snares)
Dimension Pro -2 (= Multi Toms)
Dimension Pro -3 (= Cymbals)
Dimension Pro -4 (= Whatever)
 
Now you route and re-route those MIDI note values to the specific 'drum synth' instance [DimPro 1-4 in the Out Port] that you need.  One good thing about the 'reduced track count' scheme is that you have visual feedback on your Drum Map progress (after hitting OK).  You'll have meter activity in the Track View on the 'master' MIDI track (always), and one-of-four instances of DimPro (depending on which 'drum' is struck).
 
As to why you're not quite getting there, I can't say.  It's possible that you're routing to a 'dead area' of a given DP preset.  Nearly all of the drum presets are mapped to a narrower range of MIDI notes, and some are close to (but not quite) GM mapping.  Something to check first before attempting a complex drum map from scratch.
 
Or perhaps you were using the default two tracks per-instance (one MIDI; one audio; not counting a track folder).  That may work as well, but ... I've always broken it down to the least common denominator. Avoid clutter, confusion, and redundancy.  The system(s) that I've described work here, without fail.  That doesn't help in your situation, but maybe something here will trigger a thought, or point you to the right direction.