It would be so much better if you had copies of letters you had written to the landlord telling him about the tree situation, to prove that he had been aware of a pre-existing problem. But hindsight is 20/20. At this point all you could do would be to call in a certified arborist to give an opinion about why the tree fell. If the tree fell because of high winds, then no one is to blame. If, in the arborist's opinion, it should have been obvious to anyone that the tree was a hazard, then you might have a good legal case - which you will still have to drag through court.
If the tree was rotten but you can't prove the landlord knew about it, then he wouldn't be responsible for the damage, but he would of course be responsible for the cleanup. I'm not sure why the landlord's homeowner's insurance won't cover the damage to your cars, but his insurance company must be asserting that it was an act of God or something.
If none of these options sounds right, then you'll have to let your insurance cover the Altima and risk your rates going up. The antique car that was not insured (what was that about??) is something you are going to have to pay out of your own pocket to fix, if you decide to do it.