It certainly seems like it can do the job. I'll try to help give feedback but I'm still a little confused about the exact rules for diplomatic ships though. Is it necessary to have literally no weapons or just for that ship class to not have fired them in the presence of the particular aliens you're talking to? I would think that having some limited PD/self-defense capability might save the ship if negotiations collapsed and they needed to run for the JP, or if they got attacked by a third party on the way to or from their station. It might be thematic and even useful at times for a diplomatic ship to have a marine detachment on board as embassy guards as well. But if you're going to assume the ship will die if attacked, maybe instead of making it less fragile, it should just be smaller and more expendable? You could build cheap diplo stations and tow them into orbit around alien colonies with a commercial tug, or maybe ferry around small-ish diplomatic shuttles with a commercial hangar transport, without giving much offense to the locals, and then you'd lose less of an investment if they turned on you.
If for threat determination purposes aliens subtract 10,000t from the class of one diplomatic ship whose class has never fired weapons, and then they also treat any ship without military engines that hasn't demonstrated weapons capability as 10% of its tonnage, do those things stack somehow? How does the math work here? If you subtract 10kt first and don't multiply tonnage by 10% because diplo ships are a special case then 11,000t is the magic number, but if you subtract 10000t first and then multiply by 10%, a diplomatic station or commercial-engined ship can be 20,000t without causing additional concern, which could let you build a significantly larger and more capable ship or outpost. If you multiplied by 10% first and then subtracted the 10kt then the magic number would be 110,000 tons - there's no way it works that way, though, right?