Can I ask the moderators of this instance to delete my posts without a trace so that they are not in the federation database?

A instance admin can only delete your posts from that specific instances database, so you should assume that there will always be some sort of copy of your post (with things like search engine caching and the Internet archive this basically the case on any social media).

Is there a way to chat safely on Lemmy? Alas, I like to communicate with different people from different countries, because I can’t help myself, by the way, how then can I pass the keys to my matrix room to others or how can others give their keys?

Not a active matrix user, but the default Lemmy chat is definetly not secure. Lemmy allows you to add your matrix handle in your profile settings so it will appear when someone looks at your profile

Quote from the Open Source Initiative definition of Open Source:

The license shall not restrict any party from selling or giving away the software as a component of an aggregate software distribution containing programs from several different sources. The license shall not require a royalty or other fee for such sale.

Source

Tying it to revenue wouldn’t work that well due to inflation. Metas AI has a license that basically says that, but with a user number. Both ideas however would mean that the project isn’t open source anymore