You can determine which DV-AVI type they are with
DVDate
If you have trouble with the current "type", use DVDate to change them to the other type. then try them in your editor.
A (now) free program that will allow you to do basic cutting/trimming of your DV-AVIs is
Scenealyzer. You can't add titles or do fades but it will allow you to save the trimmed/clipped file into DV-AVI (no loss of quality) so that you can then render to MP4. There are plenty of "simple" options for that, including Handbrake.
At this point, you certainly don't need to re-capture.
Re your comment re Windows 10/Firewire, if you have a Firewire card, you wouldn't use Elgato (which would re-encode into a non-DV-AVI format), you'd use
WinDV (which simply "transfers" the video, in Digital form, to DV-AVI files on your computer. In any case, I doubt whether that would be needed; you've already captured the tapes, hopefully into DV-AVI.
Re the Apple, that would certainly be an option. You quite possibly could just put your DV-AVIs from your XP box onto a external drive (NTFS?) and put them on to the imac.