You're going to get a lot of divergent opinions on those questions, but remember, it's YOUR car. Unless you have plans to sell it at a specific point as an investment, all that matters is that YOU enjoy it. Everything you have suggested are in my plans to do as well.
If you are looking to maintain investment value then maintaining as much originality as possible is generally more important with Mangustas than Panteras. New headers and mufflers made to look like stock should be fine because those are "consumable" items that do get replaced over time. As far as I know, Mangustas were not shipped with any consistent type of side mirrors (some, including mine had none). Paint changes seem to be done often enough that I wouldn't worry too much (I plan to change mine when I paint it). Euro side lights are at least DeTomaso correct, so I agree there too. As far as a Holley carb goes, I guess if you aren't switching to Webers it's OK.
I think as long as the changes are consistent with what was available on/for the car at the time it was built then you aren't likely to offend anyone but the utmost purists.
I hope the repairs are going well and look forward to seeing what you decide to do for color. I've changed my mind 2 or 3 times already on my choice and I've not even started doing anything yet.
Mark