Maybe tether is supposed to replicate the Sha-ir gen servant, but it's a clunky workaround that the Pact Boon isn't given until 3rd level, and the next opportunity to give an otherwordly patron feature to alter it isn't until 6th level.
I do like the idea of the Sha-ir being a warlock (2nd edition didn't have these other spell caster classes). But I think the best thing to do would be to make a minor gen servant
I would then alter the Collector's vessel to "Gen Vessel" keep it as a spellcasting focus, allow you to "release gen servant", and have the gen servant have an AC and Saving Throws equal to yours, beastmaster's beast style hit points, but you can cast spells from it's square and boosts your perception and arcana for an hour (I'd add arcana to give a little more of that consult gen for spell knowledge feel). If it reduced to 0 hit points it returns fully intact to your vessel.
I'd then add a pull box with fluff that connects your gen to your boons (the gen servant fetches you a weapon from the genie's horde [blade] spells from the library [book] or animals from their menagerie [chain])
Protective wish would then be more clearly turning the minor gen into some extra hit points for you (hence the Beastmaster pet HP progression)
With maybe an Eldritch invocation for chain warlocks to get an elemental servant OR include a stat block of a greater gen servant and have a box "at the DM's option a Warlock pact of the chain can select a greater gen servant as their familiar).