Based on your descriptions you should be arriving at 12:05 on the FI service, and want to connect to 13:05 LH service, on a day when there is no 5 pm LUX flight, only 21:40.

1 hour connection on separate tickets, even if on single ticket from T2(E) to T1(A) would be a stretch. And even if you made it, you still have to travel overland to Metz. At this point, I am really unconvinced a train connection is that much more convoluted, despite everything.

You do have one, somewhat slow, but very convenient option - there is a Flixbus coach leaving for Metz from the airport at 3:05 pm. Takes a long time, and is a coach, but is a direct connection.
Train connections, alternatively can take a bit shorter or about the same, but come with two connection.

There is one which would get you into Metz in 3.5 hours, which is competitive with the plane to LUX and then overland, but unfortunately leaves at 12:51 which means you have the same issue as with the LUX flight, not enough transfer time. Multiple options leaving around 2 will get you to Metz at 6:15 or so.