simonev,
welcome to
RP.
you're right, it's an interesting piece. i'd post this in the history forum on retouch. you may get more responses and attract those that know this stuff better than i.
to my eye it appears to be a photo that's been colorized. the woman looks to be colorized not in oil, but the background certainly is.
as to how to take care of it, i'd say if you have a VERY fine, silky type lens brush, brush it very lightly with that. and, i'd get a uv glass covering put on it to protect it from the light. you may also want to do something about humidity/moisture/mildew. and on that, i'm not quite sure.
i'd also have the whole appraised. it may be of some value besides sentimental.
i cant see the glass it's painted/photographed on, so cant really comment on that.
also, your last link you posted gives a login page, so couldnt see anything there.
if i remember correctly, when you photographed on photographic glass plates, the image was a reverse image or negative, but i also seem to recall that some of those might have also been reverse negatives (positives). but not sure of that either. do check the history forum. the folks that love this stuff will probably be able to help you better than i.
craig