I would like the facility to be able to give individual user permission to pin and unpin topics without having to give them full moderator rights.

May not be exactly what you're looking for but this could be accomplished already on an individual forum basis by making the user a local mod of the forum. You are able to pick and choose how much "power" the user will have and one of the options is pin/unpin. This allows you to have users performing mod actions without full mod rights.

Would I be able to restrict the user to not see invisible posts/topics and also not see the warn level?

Just tried it and it works a treat, many thanks.