As far as going to the ohel there is no davening to the Rebbe ch”v. When one writes a Pan (or kvittel) the minhag is to start off with the words אנא לעורר רחמים רבים . We ask that the Rebbe should intercede on our behalf and be מעורר רחמים from Hashem that’s how every kvittel starts.
As for the permissibility of this the מנחת אלעזר (who is not a lubatcher or biased according to some posts here) writes this is not only permissible it is a MITZVA. See שות מנחת אלעזר סימן סח look it up for yourself please.