The reason that you can't pass cons as an argument to foldl1 without casting is that the type of cons simply does not match that of foldl1's function argument and your cast is in fact illegal - but, sadly, unchecked, so you don't get an exception right away (you will get one eventually though).
You've defined foldl1 to take a function that takes two arguments of the same type and cons doesn't do that. cons takes a T and a list of Ts. When foldl1 first calls cons, it will call it with the list's first and second elements as arguments. Since neither of those is a list, this will cause a ClassCastException to be thrown.
So cons is simply not a valid argument for foldl1. Not only is there no way to avoid the cast, there's also no way to make the cast actually work.
Another reason that you can't use foldl1 to reverse a list is its return type: foldl1 returns a T where T is the element type of the list you're calling it on. So if you use foldl1 on a list of Ts, the result will be a T. But clearly the result of reversing a list of Ts should be another list of Ts, not a plain T.
If you want to reverse a list using a fold and cons, you'll need to define it in terms of foldl, not foldl1. foldl allows for the result type to be different from the element type of the list and it allows you to use something other than the first element of the list as the starting value of the accumulator (which you need because, as I said, the accumulator needs to be a list).