In an authentic hadeeth the prophet peace and blessings be upon him said "A strong believer is better and more beloved to Allah than a weak believer, while there is good in both"
The problem is that some who comment on these issues dont understand martial arts to begin with. And there is a rule in Usool Al Fiqh (Fundementals of Fiqh) which is that "Judging/giving a ruling on something, is a branch from how it is percieved"
So if it is not understood well, the ruling will not have a solid basis.
You're young anyway, and will need to build your base. So even if in doubt, you can start by learning Brazilian Jiu Jitsu (gi and no gi, not just no gi) , and maybe add some wrestling for takedowns. These dont involve striking, and grappling is the backbone of MMA, and its what really takes lots of time to learn.
Grapplers can improve their striking skills fairly quickly vs strikers who want to learn grappling (they'd pretty much have to start from scratch). You'd have a solid base.
Also, from my experience, MANY MMA gyms, teach everything but are Masters at nothing, and basically are in the business to make money. Because when many see MMA fighters they say "Oh I want to do that", so they say "we'll teach you MMA". Its all marketing.
Some might disagree, but I think having a very strong base in Jiu Jitsu and wrestling, is the way to go. Some MMA fighters try to stand up the whole time when fighting a grappler but thats not really practical.
Anyhow, I personally think grappling would be the best way to go, its both safer (not involving strikes) and its what it takes to be a good MMA fighter, but since it takes lots of time most want to skip it. So you have an advantage being young, having time to learn and be really well rounded.
I personally dont understand those who think its not permissable, especially that chances are, they are healtheir, and less prone to bad habits like smoking, or bad eating habits that can lead to diabetes, heart disease, and so on. In addition to the points you mentioned about boxing which makes it much more dangerous than MMA.
By the way theres a VERY good Jiu Jitsu black belt in the US who is not only Muslim but a praticing Muslim upon the Sunnah.
As I would advise my younger brother, just dont let it keep you away from your studies. School comes first, because education is what will guarantee you a reasonably good future insha'Allah. Other routes cannot guarantee you anything. So keep it as a hobby for now, and focus on your studies. You can do both, just dont let your hobby distract you from your education.
And if you do decide on Jiu Jitsu, just find good laid back gyms where theres no real bowing and so on. Most dont involve that (most Brazilians are laid back) anyway unless if the instructor has a Judo background as well.