Telumë will be very open with them about everything he knows.
Haven is extremely well guarded. Maitimo is going to be expecting a rescue attempt, and it's also the case that, while Telumë can recognize his style in the decisions made by the new management of Valdemar, he isn't leading openly. Probably because it's a bad look given Vkandis' stated goal of preventing inter-world contact; also it means that Maitimo approximately doesn't do public appearances, reducing their opportunities to snatch him. He does occasionally visit Iftel and Karse, but usually has mages Gate him rather than travelling overland; Telumë isn't sure, his spy coverage is spotty in Valdemar and Karse and nonexistent in Iftel, but he suspects the Gates are directly between temples to Vkandis. Vkandis has the home-ground advantage on literal holy ground dedicated to Him.
Vkandis probably doesn't have that advantage in Valdemar proper. The coincidences are subtle but there, and usually inconvenient for Vkandis' goals. Vanyel suspects the god behind the Shadow-Lover's facade is responsible. (He's offered to arrange another near-death experience in order to confirm this, but been vetoed by everyone else in k'Treva including Melody.)
Gating directly into Valdemar is out. Maitimo knows exactly who should be Gating where, when, and unauthorized Gates are responded to in force very quickly. Telumë has tested this. Entering from the west is an option, but a risky one, given the current tense stalemate between the Tayledras and remaining Valdemaran resistance forces and Vkandis' side.
Entering from Rethwellan might be the best bet, if they can arrange to get there in the first place. The border is less thoroughly guarded, since Maitimo is clearly wrangling for friendly relationships and mutually beneficial trade. And Leareth's organization has spies there. They've gone underground rather than fight and risk exposure, but if they're willing to trust him, Telumë can give them contacts, including with merchants who might well be able to smuggle warm bodies into Valdemar.
...Not mages, though. It's extremely difficult for mages to enter Valdemar and particularly Haven, there are people who know how to test for Gifts checking, and Sauron can show up in person to cancel their magic and murder them. Also, any plan that requires Vanyel is doomed to fail, even if they can somehow get him back to Arda and healed. He's the most expected rescuer.
Telumë isn't sure how one might stage a rescue without relying either on Arda's magic or Velgarth mages. He is very carefully not going to ask what Arda has in reserve right now.