The double-quotes around $i are un-necessary in your first line of code.
Correct. Just like you can do a print $i vs print "$i".
This doesn't hurt and I think it makes it more clear that this is a string. I figure this is a minor quibble.

A space after the '<' is allowable in your second example
Correct. just saying that it is not *required*
I've never used the 4th form in production code, my mistake copying without verifying from another post. I personally have never used it. Ooops.
Thanks for the clarifications!