If a value in a cell doesn't exist in it's corresponding combobox, is there a way to get around iGrid showing a blank cell?

Access has an option "Only from list" for comboboxes which you can set to false so the user can enter whatever they like in the combobox.

Is any of this possible in iGrid? If not I'd really like the feature.

No, iGrid combo box cells can display items only from the attached combo lists.

Good news: this functionality is available in iGrid ActiveX 6 now.