I've got the same scenario: Their home system, their suggest leave system next to it, my colonised, suggest leave system next to that. So far parking my diplomatic ship in their suggest leave system is gaining plenty of diplomatic rating and they seem content to tolerate it sitting there indefinitely.
An option to give diplomatic ships access to your systems but not military ships would be a good improvement. We are getting along well enough now I am tempted to remove the suggest leave status on my neighbouring system, but I am worried they will colonise bodies in that system, and others, that I plan to use eventually.
FWIW, the diplomatic rating increase that shows in the event log comes from THEIR diplomatic ship talking to you. THat's why you are getting that even without a diplomatic ship on site.
Thanks for that info, I think that was one of the main things I was misunderstanding. Does that mean if my diplomatic ship increases my diplomatic rating with them, but there isn't a diplomatic ship of theirs in the area, that I can get increases with them but won't have it reported in the event log? This is where I seem to be having trouble increasing diplomatic rating with the two other NPRs.