Yes, the neighbors do visit. Baako and Billie Jang are goofballs - so come in and make themselves at home. But, I have not observed them in my sims apt while visually sick. Mostly I see them with visual ailments in the lobby or when complaining about the noise.
None of my Sims have gotten sick. Which seems odd. At least one should have a chance of getting an ailment over the 2 sim weeks I've been playing. Especially because they interact with the sick neighbors. I don't have ailments/luck disabled.
The restaurant was NPC owned. I assume that the gnomes invading the restaurant because of a sick waiter was normal for EBN.
This is the first SM apartment I've played since EBN. I haven't played a r-rental setup yet.
I'm not sure what the range is for ailment effects. Apartment and r-rental work differently from normal lots so maybe the effects are spilling into my sims apt from the one next door. Need to do some more playing in a different apt. and some r-rental.
If it keeps happening in other SM apartments or r-rentals, I'll make a bug report.
Adding: It seems to me that when I was playing my test game right after EBN came out, my Sim got some sort of warning about making the gnomes angry. Do gnomes not like Spellcasters using Magic? Do they keep showing up because my Sims use magic. 🤷♀️