Until now Mervos was totally unknown to me but as soon as I saw your pic UG came to my mind, in this case their Triple Calendar. Furthermore the connection between the brand is confirmed by the 9TH picture you have posted a (I guess) oversized Mervos, which is almost identical to a very rare oversized 46mm UG, which was issued to Pilots and is published in Papaleo's New book "FERO".
Talking about the calibre IMHO the fact that UG gave to Marves some "leftovers" makes sense, but as you pointed out, the connection went beyond that. They shared the same case maker, the caseback was probably adapted from an UG one and also the soleil finishing for the dials.
I enjoyed reading your research a lot, thank you for sharing your knowledge and passion with us!