A tracker will not give more mp. It allows for longer exposures without trailing.
Stacking reduces noise but doesn't give more mp.
You can do both stacking and tracking if you want for longer acquisition time and lower noise.
If you are shooting a panorama then you don't need to use the widest lens ie a 20+mm lens can have less coma than a 14mm lens.
Coma is most prevalent in the corners
If you use a single shot (panel) then you cannot avoid it. If you take multiple panels and overlap by 30% then only the centre portion of the image is used. The other pixels are used for matching during the stitching process and then thrown away hence the ones affected by coma. For a single row, the remaining pixels affected by coma will be on the far sides and top. No coma on the bottom as it will be the foreground with no stars. If you shoot enough panels then you can crop the sides and top and have no coma.
When stitching using PTGui (the best stitching software), you get a lot more pixels. For a 14mm lens x 7 panels for a single row of 7 shots on the R5 (milky way bow), I get a resulting ~600mp image after some cropping.
Using a A7Sii, a single image (stacked or not, tracked or not) will give you 12mp with low noise. Note that having larger pixels helps with longer exposures as trailing is less (field of view per pixel is lower with less pixels).
In your scenario of a single shot, you can take more images eg 1 row of 3 and then stitch them and crop back to the same field of view. This will remove the pixels with coma for the sides. If you were originally shooting landscape orientation then change to portrait and crop for the top as well.
If you use a 20+mm lens and then stitch then you get more pixels for the same field of view as a 14mm lens.
The question is how big you are printing as you may not need more than 12mp. Whether a viewer will ever see the coma in an image (social media or printed) is also debatable vs pixel peeping at 1:1 on a screen.
You can track and get 2 minute exposure. Stack 2 of them for 4 minute acquisition time and then 7 portrait panels. 20mm lens with 30% overlap in portrait orientation gives approximately 180 degree field of view horizontally when stitched. Panorama for the sky would take ~30 minutes. Another 7 shots untracked for the foreground. Acquisition time will vary depending on how much light is available and whether taken during twilight or at night (with or without artificial lighting).
Trevor Dobson has been using 85mm for milky way panorama stitching 210 images!!
https://www.flickr.com/photos/trevor_dobson_inefekt69
Hope this helps.
