We are a group that plays a lot of melee and we have some questions about what we can do or not. The cards are as follows Tithe, Margaery tyrell and highgarden.

First Tithe, can you kneel opponents neutral cards with this.

Margaery tyrell. Can you boost other players characters with her.

Same with highgarden, can you pay to remove a attacking character even when you are not getting attacked.

Whenever you have a card that says "do X" to "stuff happens", the first part is a COST. The word "to" is the giveaway here; Margaery and Highgarden follow this templating as well. You can't kneel an opponent's character anymore than you can use an opponent's gold to activate Highgarden. See "Costs" in the RRG.

Margaery does not stipulate "a character you control." She's awesome in melee.

Highgarden also does not stipulate anything other than a character who is attacking in a challenge. Also awesome in melee.