3, 25 is not a concern, many of bihari, mh, assam, there are lot more just cannot remember who are average 24, max is 32, many are even married, saddest part is couples are studying
4, but given that you completed CA if you opt to go college its must be full time so its considered a break for your current profession, so choose AMIE, i hope you know what society means or contact them for any query
its associate membership for institute of engineers India there is also AMAeSI its associate membership for aeronautical society of India
but its necessary to do DIPLOMA 3 years anyhow, instead of 6 year profession break im advising you to take only 3
the main advantage of society is no college can run this course, so you have to study by yourself or join institutes, you can work in day and study in evening still its regular, you will get degree directly from society
a final heads up, if you want to go abroad to settle its a necessity to obtain the appropriate society from the government such as AMIE for BE/BTECH or IEI for ME/MTECH and other dep such as CA or MBBS too
so im just telling you to directly do that and save time, money and profession break
but DIPLOMA in mech in regular is what i suggest before BE or AMIE
" and its universal for all other non PCM in 12th aspirants too"
good luck, this is my first answer in three years and its going to change ones life ha ha ha