It's not about guaranteed placements. You send a tip to be checked with priority which highly increases your chances to be posted, but still no guarantees.

Only one: one song per submission. If you've sent more than one, be sure that only first song link will be checked.

Kinda. If you send tip submission – there's a chance that you'll be added in more than one playlist. Your song also may be added more times later, after the first publication.