ListAnimate[y2] makes me think you expect Do to be able to work like Table and produce a list of results of each iteration. It cannot. Return could return a single result the first time it is executed (a.k.a. "evaluated") in the Do loop. For it to return a plot, the argument to Return should be a plot. But the argument is not a plot in any of your examples.

Further, you place Return[..] as an argument to Plot[], in a position where Plot[] expects an option. I get error messages, which seem to be ignored in question.

If you want to make a list of output (plots or whatever), use Table or Map. They're the best ways. If you want to make Do[] construct your list then try something like this:

resultList = {}; (* initialize *)
Do[ nextResult = ...; AppendTo[resultList, nextResult], {...}]

Or this:

resultLL = {}; (* initialize linked list *)
Do[ nextResult = ...; resultLL = {resultLL, nextResult}, {...}];
resultList = Flatten[resultLL]; (* does not work if nextResult is a List *)

The second, linked-list way is usually more efficient than AppendTo.