Good morning,
1. I would ask the landlord with not too much details; some landlords can be greedy. I would ask if it is ok and let the landlord know that at times when you are traveling, not sure when those dates are, you want to pay rent but can't afford it so subleasing is the only practical way. Never offer a commission to the landlord, they usually are greedy, sneaky and intrusive people. If they get pushy it might not be worth it. If they say yes but seem suspicious, call the Detroit housing association for some help.
2. If you own an apartment building you have the power to do whatever you want. A renters policy through your insurance should cover this. Call your insurance company to see what they can do to cover you, they can give you an estimate.
3. Again, you do not have to have an LLC to own an apartment building unless your state requires it. You can own a building and INSURANCE is what covers all liability. You would probably need a commercial business policy and/or renters policy to cover all angles. I would get a quote from your insurance company. If you have American National Insurance call them, they are fantastic!
4. Owning an apartment building vs renting out spaces is very different. If you own an apartment building I would use an LLC. You can register your business through the state and then get the appropriate insurance for coverage. If it is just you renting out spaces/places, I would keep it in your name and sublease under your name. If you go this route make sure you have a tenant policy to cover damages, etc.
I hope this helps! Good luck