so it will be no surprise that there are palantiri on Tol Eressëa.
How? Toll Eressea is a stronghold of the Teleri - those should be reluctant to use anything Noldorin and grossed out by their "technology"? Before the War of Wrath there are no other elves there than the Teleri.
It seem that some Vanyar were there after their return from the War of Wrath - before they are allowed re-entry into Valinor proper - if we follow the idea of Ingwion having build Kortirion there, but still even if we assume some of the Vanyar having been stationed there did tarry longer, the Vanyar themselves are not into technology much.
So the only way is if the Vanyarin army has used some Palantiri (given to them by the Noldor of Tirion at their depature for thr War of Wrath) during the War of Wrath in Middle Earth and just leaves them in Toll Eressea after its return and having no other uses for them the Toll Eressean gift them to Numenor - as humans like any technology and always find uses for it.
Has there been any reconciliation after the kinslaying in Valinor? We know Teleri will not join the War of Wrath because they still hate the Noldor while the Vanyar do go. Anyway, the question why and how does Toll Eressea possess Noldorin artefacts created on the mainland of Valinor in just a couple of centuries to be able to give them to Numenor needs some explanation - it is far from obvious. Also we will soon be there when Earendil arrives - we do need some thought about the current state of Valinorian politics.
The technology we will be developing in Gondolin at this time will be the swords-glow-blue and how-to-kill-a-balrog varieties.
Why do they bother with it if they rely on secrecy and have little reason to suspect any interaction with any Balrog in the next dozen of centuries?
Was not their aim to create a Mini-Valinor in seclusion? Why would they focus on military technology? What do they plan to do with it? Is this a secret project of someone having foresight - just iike Idril's tunnel? Does Turgon know about it at all - this seems not to be his own plan?