Salam Ahmed,
In the following,
But if good comes to them, they say, "This is from Allah "; and if evil befalls them, they say, "This is from you." Say, "All [things] are from Allah." So what is [the matter] with those people that they can hardly understand any statement?
I think, Here Allah is rebuking the fleeting attitude of some believers, they used to believe in Allah when good used to come to them, and blame the messenger for the evil that used to befall them. These people used to pray and believe as long as good things where happening to them, but the second a bad thing happened, they used to turn on the prophet and blame him saying that bad things were happening to them because they believed in him... that's why we see Allah clearing the blame by saying ( Say, "All [things] are from Allah." So what is [the matter] with those people that they can hardly understand any statement?)
proving that all good and evil things with which He wants to test humans and jinns is from Him, as also mentioned in
[21:35]
Every soul will taste death. And We test you with evil and with good as trial; and to Us you will be returned.
In the following,
What comes to you of good is from Allah, but what comes to you of evil, [O man], is from yourself. And We have sent you, [O Muhammad], to the people as a messenger, and sufficient is Allah as Witness.
Here, I believe Allah is talking about the inherent natural inclinations of men, we all know that a soul is made balanced and we do have free will which gives us the option to do evil things, and our souls do understand what is good and what is bad for it, but our souls do have both good and evil inclinations. Here Allah is saying that whatever good inclinations and actions we do is from Allah, since he guided us to do the right thing, so whatever good comes our way as a result of choosing to follow Allah's law comes as a blessing from Allah. But if we use our free will and choose to fall pray to the evil inclinations of our souls and willingly not follow Allah's law, then we are going to do something evil by choice, ergo evil resulting from ourselves. Now, from this conscious decision to do a evil action comes from ourselves and whatever evil results from it is also upon us. Hence, good comes from Allah ( as a result of following Allah's commands) and evil ( from within us, choosing to not follow Allah's commands).
Another point to be kept in mind is that since we are part of a society, so majority's decision to not remain honest also affects our fate. So, the concept that bad things only happen to bad people is not really right. If you are living in a place where admissions to good university depends on the amount of money you can give as bribe to the university authority, then no matter how good a student you are, you have to give bribe.But, Allah has said that He is not going to punish a person for evil things that are out of that person's control, and also if he is forced to do evil.
As far as disabilities are concerned, animals are also born disabled sometimes, why do we think that being born with a disability is a curse or a test? Animals are born sinless and they won't be brought to justice on the day of judgment, so why are animals born disabled? Are they being punished? plus humans are all born sinless, it might be that Allah wants to test the parents and people around the child and also the child if he/she is not mentally challenged. Having a mental disability that robes a person of his/her sense of right and wrong can actually come as a blessing because that person then remains sinless throughout his/her whole life.
All creations with free will ( humans and jinns ) will be tested with trials, to see who does not waver from Allah's path. we forget that great wealth , intelligence, beauty or power can also be sources of trial, even though they are perceived to be positive things but so can poverty, low intelligence, lack of beauty and power be used, ( these things are generally thought to be the cause of most evil actions brought about by men) to test a person's true level of gratitude towards the creator. All things good or evil comes as a test, and as long as you follow Allah's command regarding the matter, you will pass the test. It doesn't matter if the evil comes to you as a result of your own previous evil deeds or not, if so , you can atone for them this time by remaining patient and following His commandments. e.g. if You are rich, you have to give zakat, etc and if you are born poor or became poor because of bad business decisions, you still have to remain honest and have patience. If in case A, you don't give zakat, you fail the test and in case B, if you don't remain honest and have patience and decide to rob a bank, you also will fail the test.
Hope this helps you.