Your assumption is right, no do-overs.
I think the easiest way to enable another try would be to change the 'maxruns' setting in missionScripts_XCOMFILES.rul. Search for 'STR_DOSSIER_' to find the missions. Don't forget to change it back after you've succeeded once. Or just increase the counter by one each time you fail, but new games will still require resetting the value.
I suppose one could also modify the save, but I'm not 100% sure which entry controls reruns, and you might lose your record of previous losses this way. And it'd probably be a little more complicated than changing just one value.
In principle, it's possible to lose research, items, even bork your save to a degree. So far, the recent updates have been largely additive and the last big loss I recall was when the new Turbolaser Sniper ate your old Sniper Lasers.
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Solarius has been pretty good at maintaining compatibility, unless you're trying to play a 1.3 save with 3.1, or something like that.