What do you think the tab icons up top mean? - Home; Unknown; Unknown; Save; Print. They're not all intuitive, but once you tap them, you realize what they do. I think it would just take familiarization.
Are there tabs you don't think are necessary in the mobile version? - I'd say Save; Print, but if you want the same functionality of the original site, they would be necessary.
Are there tabs you think should be added in the mobile version? - Not known at this time.
Is it clear how you add/remove an unit from the army? - Yes
Is it clear how to add/remove an upgrade to an unit? - No, though I clicked the icon and figured it out immediately. I don't know how to make that more intuitive though.
Is it clear when units don't have upgrades? - Yes, but only once I realized what the icon meant.
Is it clear how you see list errors? - Yes
What should happen if there's no errors? - Green icon with a checkmark
Is it clear how you find list building info (points, min/max) about a unit/upgrade? - No, and mouseover won't work on tap devices, so you won't see the "?".
How should unit stats be presented? - Tap the unit and an accordion expands to show stats. Tap again and accordion closes. A popup would work as well, but would only really be useful on a tablet - smartphone screens would be somewhat small, though I have seen it done.
Other stuff I'm not thinking of? - A working sample might make it a bit easier to determine that.
I'm assuming that you're only coding two versions, and the mobile version will simply expand to screen size with CSS?
I'll take a look on my tablets and phone tonight and let you know what I find.