What does return; (without value) mean?

To answer your first question: I assume this code is decompiled. Note that decompilers are not one to one converters of binary dex code to whatever Java code has been used to generate these binaries. They often have problems with parsing different control structures like loops and switches. Besides, the binary may be obfuscated, meaning that after compilation the binary is modified in a way to be fully operational, but harder to decompile and reverse engineer (basically to prevent what you're trying to do here :) ). They can add dead code, like return statement that shouldn't be there, mangle loops and if statements to confuse decompiled code, etc.

Yes, that is correct. Look at the bottom of http://docs.oracle.com/javase/tutorial/java/nutsandbolts/branch.html:

The return Statement

The last of the branching statements is the return statement. The return statement exits from the current method, and control flow returns to where the method was invoked. The return statement has two forms: one that returns a value, and one that doesn't. To return a value, simply put the value (or an expression that calculates the value) after the return keyword.

return ++count; The data type of the returned value must match the type of the method's declared return value. When a method is declared void, use the form of return that doesn't return a value.

return;

If the method returns void then return; just exits out of the method at that statement, not running the following statements.

return keyword is used basically in a void method ,it helps to break the conditional statement to come out of method.