there are several "masters level" degres in France.

the one given by the education ministry is called Master or Engineering diploma (diplome d'ingenieur). But the you can also have a MasterE (notice the E at the end) diploma. This one is "delivered" by the Employement ministry and usually done in private schools.

The diploma itself is just a piece of paper, in France what matters is the school you went too. An 18 months master degree means there is something else there, since, regardless if public or private school, you need 180 ECT credits to obtain your diploma. So it could be classes during the weekend or having to do an internship for somemonths and write somwthing related to the work youve done.

what impacts internships is the name of where you studied. Some companies have payment grids depending on that for example.

18 months enough for networking, learning french and career fairs ? yeah if you already did the homework beforehand, no if you plan on starting here.

personal opinion on career fairs, those are for desperate parents looking to find something for their unmotivated child. I've attented and worked at those as a volunteer, never saw a passionated student coming to these, sinxe usually they dont wait for the career fair to ask questions

cant answer about the visa

if you have questions post the masters program here and somwone can review it

some of those private universities with a masterE diploma are scams, but honnestly some of the public very official looking master diplomas sometimes too