In answer to your main points:
1. Although there are no formal qualifications necessary to obtain a professional licence, an airline sponsor most certainty does require at least sixth form subjects, if not a qualification in higher education.
So to be capable of passing the ATPL exams, a basic education is a prerequisite.
In the UK, that would translate into a minimum of 5 'O'/GCSE/NAT5 including Maths & English. Ideally Physics as a third subject and at least two other sensible acedemic subjects. Nothing remotely connected to "under water basket weaving" subjects. Then sixth form 'A' levels / Highers / AH preferably in science although not critical provided good grades (A or B) are achieved.
2. Integrated more expensive than the modular route: double the cost often quoted
3/4. Most unlikely in the civil world, although the military support education prior to adulthood.
Complete your education and then contemplate your options, unless the military route appeals to you.
4. You are dealing these days with beancounters, who know the cost of everything and the valuable of nothing.
The military route might offer this option.