Based on the Motolite reference, the frame sounds circa 1983/1984. Is it inexternal brazed? While I have no objective evidence to support this, I believe that Motolite was a manganese alloy tubing. I have a couple reasons for this. First, the lower Nomade used still used 2040 during this period while the higher Super Mirage used CrMo, implying that Motolite fell between 2040 hi-tensile and CrMo tubing. With the exception of Reynolds 531, most tubing manufacturers postioned their manganese alloy tubesets between their hi-tensile and CrMo offerings. Secondly, this was era where the lower priced manganese alloy tubesets (ie Mangalloy, MangaLight, etc) first started to appear.
Motobecane definitely offered mixed tubing frames. However, the common practice was to use better tubing for the main traingle and economize on the rear stays and/or forks. For instance, the 1984 Super Mirage has a tubing label that specifically says "3 tubes CrMo'. and boom era models, such as the grand Jubile, used Reynolds 531 only for the main triangle.
A seat tube is in the middle of the frame and would have relatively little affect on the weight distribtion, as opposed to a rear triangle made out of thicker material. However, the tubing type of the seat tube can be assessed, to a certain degree, based on the seat post diameter. The premise is that stronger alloys permit a thinner tubing wall thickness and consequently larger seat post diameters. Most hi-tensile steel seat tubes use 25-26mm posts.