An interesting conundrum.
If I read you correctly, you can go direct entry pilot, or you can go and do your MechEng / leading to Aero degree - but apparently not both at once. Well, look at it this way, you can't do military flying training in your own time by correspondence course - so frankly go for it. If you fail on that route, your University entrance qualifications will remain valid and you can use them later. If you succeed, there are numerous routes, in most countries, to do the Engineering degree - Mechanical if not aero in your own time later - and if you want to go TP you'll have enough years whilst you're building up experience as a front line pilot to do that.
If you fail on the military flying route, then the standard route to civil TP (apart from being ex-mil) is...
(1) Aero Engineer
(2) Migrate into work as an FTE
(3) Keep your flying up, progress towards CPL
(4) Eventually migrate from FTE who can fly, to TP.
This is a slow route (like 8 yrs +), but all of the steps on the way are interesting, and most of them pay. It was my route in, and I'm sure that of a lot of other people around here.
G