I've investigated the "can't type a negative number" thing, and it's Windows' damn fault. Note that this is completely stupid, by the way. Here's a little trick you can do:
1. If the value isn't already 0, enter 0.
2. Click on the down arrow to make it -1.
3. Edit the value without deleting the minus.
Once some versions of Windows you could even use Ctrl-V to paste non-numeric values, but apparently this doesn't work in Vista.
The issue can be fixed by editing the wxWidgets library and recompiling it before building SLADE3. Or potentially by changing the property in the relevant constructor to add wxSP_NON_NUMERIC to some sort of style or something, but I'm not sure how to do that.
See
this for further information.
I agree 100% that mass-import will need to be put back in by the way.