Hey Chris, I do believe adding Milestones to the Add-In for Excel is on the roadmap, although I can't say when! What I can say is that you can always add this idea as a feature request on Rally Ideas (
https://ideas.rallydev.com) to garner votes and discussion!
If you end up doing that, share it back here!
As far as the issue with the custom grid, I recommend submitting a case with Rally Support to look into the issue to determine whether there is a defect there.