Let me make some things clear, I am not a writer on lost, I never was, I am a fan only. I do not know of secrets told by jj, carlton and damon. I am giving these answers based on opinion and facts and speculation, I do not know if these are the actual answers but I figure they would be. Also I am not going to answer most things about the flashbacks cause I skipped most of the flashbacks and have no desire to ever see them. Lastly, I will try to remember every single unanswered question, every loose thread and again this is just my speculation, but I figure I will attempt to give them out.
1.who were the people who shot at sawyer and lockes group in the future?-I can only assume they were the ajira flight people and they got their rifle from the others/hostiles
2.Who takes place after hurley, ben, walt, rose and bernard die?-To my knowledge, they were the only ones left on the island alive in the finale and none of them are immortal like jacob. We also know at least 4 of them go to the afterlife, so I can only assume the whispers if there are any left were watching over the island. Its also possible the island doesnt need a protector but I am sure if it does, there was a plan put in place before ben and hurley passed.
3.Was walt able to help his dad and the other whispers pass on and find peace?-Im not sure he did, even if he did, michael at least didnt go to the church in the afterlife, I dont think he did nor did the many other whispers on the island.
4.Why werent all the protagonists of lost in the church scene at the end?-In reality, they probably couldnt get the budget to get everyone back, in the show itself, probably there were certain characters who couldnt move on yet and find peace.
5.Was the island or jacob manifesting at walt, and possible other hallucination characters?-I can only assume it was the islands conscious and jacob was merely its like protector, its bodyguard.
6.Why did the man in black/smoke monster have the weaknesses he had?-Something, possibly the island had placed those weaknesses over him.
7.Did any of the hostiles/others survive?-I believe I heard that maybe some may have survived from the onslaught of the last season, like maybe cindy and the children got away and kept to themselves, they clearly didnt get off the island, on screen at least. Same goes for any character killed off screen.
8.How did jacob and man in blacks mom kill all the original hostiles?-She either used poison or maybe she could control the smoke monster like ben later did.
9.And how did ben and possibly the mother control the smoke monster?-Its highly possible they didnt actually control it, more like persuaded it to kill people who were a threat to it.
10.How did the mother know about the magical things of the island in the first place?-EIther someone before her told her or the island told her.
11.Who were the skeletons in the core of the island?-Possibly the original inhabitants
12.Who created the cursed numbers and why pick those numbers?-Maybe the island created the numbers and it just happened to like those numbers.
13.How did hurley and miles get their mediumship powers?-Again the island probably gave them their powers, everything magical seemed to come from the island.
14.Why did the island pick only jack, kate, hurley, sayid and jinn to go back in time and not ben, sun, frank and the others on the ajira flight? It probably wanted them to do it for favortism, it felt they could offer more than the rest of them
15.Why did jacobs lady bodyguard(forget her name right now) want to burn down his abandoned cabin?-Either she hated the cabin or maybe there was a secret she didnt want to be revealed. Unfortunately I have to go with the former as unlikely as it seems cause if there was a secret, we sure didnt know about it.
16.If jacob knew of certain things to happen in the future, why didnt he stop his own death?-I guess he knew it was inevitable and didnt try to stop it.
17.Why did that psychic tell claire if she didnt have aaron, the world would end?-Either he was making that up or perhaps aaron still has a role to play cause in aarons entire time on the show, he doesnt really do much. You could argue that because of aarons birth in which kate eventually adopts him and goes back to the island because of him, she gets the chance eventually to put a bullet in the man in black allowing jack to finish him off cause if she didnt do that, its possible the man in black would have killed jack, sailed off the island and the island would have exploded or whatever and possibly taken the earth with it.
18.Why did kate not want to tell jack what happened to aaron and where did he go?-Im not exactly sure why she wouldnt want to tell him but I can only assume she left him to claires mom to adopt or take care of.
19.What made the smoke monster and the island?(magic of the island)-I can only assume it was the originator of it all, the magic source, something probably relating to something angelic and demonic.
There might be some other minor ones like why did john have bruises on his face after falling off the building despite not landing on his face?, or did paulo and nikki survive through some magical means? These are going to remain mostly unknown but if you can all think of anymore questions that need answered, please let me know so I can attempt to answer them.