It used to be custom (in high society, not anywhere I have dimmed) to sit boy girl boy girl, and for ladies to talk to the man in their left during the first course, right during second... to keep a balanced conversation going

Is it not rude to separate couples?

That makes two of us. I've never heard of (or thank god, been to) a party where a host is forcing people to move around, especially in an unnatural way. Nothing feels like a forced party more than, well, forcing.

I think a lot of stuff is cultural. For me, I detest music at social gatherings. I'm there to chat with people, not to listen to music. Music, for me, can only be neutral at best (and more often it detracts fun), not a value add. My wife, on the other hand, considers an event "like a funeral" if there isn't music playing. Just different cultures. Sadly, it means stuff hosted at my house doesn't ever align with my preferences, because happy wife and all that.

Usually, people who don't want to go to your party will find an excuse as early as possible to decline your invitation in order to avoid unpleasantness and awkwardness for themselves.

Assume that the vast majority of people you properly invited and that RSVP you DID want to go, even if they flaked at the last moment. Yes, there are some inconsiderate bastards out there, but there's a big subset of flakers that do feel guilty/regret not going, far more than we can imagine.

You need to understand that in the great schema of life, parties for the invitee are always at a lower priority compared to work, health and family issues: A single guy may have finally got a date, a mom can be having issues with their kids at school near exams period, someone may be anxious after a not so great feedback at a work 1:1 with their boss and decided to polish their resume.

And besides life stuff preempting party attendance, there are a lot of other factors. Some people you invite may have been raised in an environment where, due to poverty, immigration, family issues, they were never really in too many parties, and thus, while they may wish to enjoy your party, they may become too anxious to attend what is an unfamiliar experience to them.

Depressed, low esteem people, for example, will have a big probability of believing that your invitation was not that serious, and that you only invited them out of politeness. Actually, you don't even need to have depression issues in the mix for that to happen, some cultures have a marked tendency to avoid directness in communication, and for those people, if you don't have a close connection to them, or if they perceive you as higher status than them, they will believe your invitation is not actual for real, and they are not really expected to attend.

So, for some of those people I think that is worth your effort insisting more than twice, maybe trying to make they really feel welcome and needed a bit more.

It may sound crazy and counter-intuitive, but sometimes, just sometimes, some of the people who flaked do respect you more than some of the people who went and just wanted to have a good time for free.

Don't take it personally; not inviting them after a few times is enough.

Sure, in an ideal situation people would naturally mingle, but a lot of people are shy or will just stick to people they know, which makes it less valuable as a social event.