Let's not outright lie to them. Webs of lies have a tendency to convolute and collapse with terrible and unpredictable fallout.
[x] Plan:
Tentative First Steps of Cooperation
Let's keep up the puzzlebox calling card. It's fun, and in-character.
Until you're ready to leave, you want to keep control over the actual manufacturing though, because it's the best stick to your carrot that you've got. Drones are better workers anyways. Afterwards, it's not like you can take it with you and friendly ports are always good, so they aren't too threatened about losing it after, either.
Military stuff will be kept minimal on either side.
-[] Answers: friendly tit for tat.
Answer the questions, but do it a bit tit-for-tat until you've got some trust going. You're going out on a limb here and leaving yourself vulnerable.
Ask them for how they imagine this going, about their ideal scenario and plans for you. You gave them the powercells already, now it's time for them to show some good will.
--[] What technology are you willing to share?
What it will take to get into space and everything I'll inevitably leave behind getting back into orbit. Given the Mechanicus is a notch or two below me on the tech ladder, that's a damn good offer on your hands. More can come dependent on goodwill and trust. Is there anything specific you want?
= Always lowball what you're paying, negotiations will drive up the price.
--[] Where are you based?
Close enough to work with. You don't have that much trust.
--[] How large is your ship?
By spaceship standards? Tiny.
--[] What kind of assistance do you require?
Bureaucratic and political cover, resources and potentially additional hands. Mostly I just see your people covering me while I build an orbital shipyard for your people to inherit. Or building stuff according to plans I lay out. I imagine that'll be mostly a matter of what appearances we can afford to give.
--[] What are your plans for the mechanicus?
Subvert, weaken or destroy.
We're ideological opposites and our interests mutually exclusive in too many ways.
Give them a rough overview of Mechanicus internal workings and politics while at it. Demonstrate technological superiority.
--[] How long is your timeline?
Spaceship construction is the work of decades, even in normal circumstances with a functional shipyard, but these are far from normal or ideal circumstances. Decades, if not a century or two of work is not unrealistic. Reaching into the third would not be surprising if things get tough.
--[] What are your intentions for Denva Secundus?
Ideally I'd have a friendly port to return to on occasion. Perhaps even a home? It's a stranger's galaxy out there and it's always good to see a familiar face and I like you a lot better than the imperium.
I didn't mean hacking the Ad Mech's nukes.
I meant hacking Aevon's nukes, to eliminate the locks and keycodes the Mechanicus placed on them, and make it so that Aevon can operate their nuclear arsenal even without Mechanicus assistance.
Let that be part of a negotiation.