I don't think that's true. Look at this figure, that doesn't have most of the potential data collection issues you mention (issues, incidentally, that I at least in part disagree with below)
"And a third said a woman was at least partly responsible for being raped or sexually assaulted if they were drunk."

That says to me "Really really really bad problem" *on its own*. All the rest just makes it so many times worse.

This isn't meant to trivialise the issue

But it does, nevertheless. It's a bizarre set of crimes - rape and domestic abuse - in that they're among the few crimes where the public perception is that they happen far less often than they actually do. Almost every other crime people believe happens far more often than it does, and that the police should drop whatever paperwork they're doing and catch the criminals. When it's rape and domestic violence, though, society seems to believe that it's generally happening far less than it is, and for some reason believe that the false accusation rate is hugely higher than for other crimes.

Just to go back to another couple of your points:
We also don't know how many people know of the same act

Well, okay, let's use British Crime Survey figures, which ask only whether an individual has been a victim. According to those (slightly older) figures, they estimate 12.9 million incidents of domestic violence (*excluding* sexual threats or force, which get counted as a separate crime) against women in England and Wales (and 2.5 million against men).

an instantly regretted slap once in a heated argument

A lot of the problem with domestic violence is that a lot of it is apparently like this. Someone loses his temper briefly, and lashes out and then is extremely apologetic afterwards. A few weeks later they lose their temper over something else and do it again. And again. Meanwhile, his partner is getting more and more fearful of when the next time will be and what will set it off (which of course, might be an announcement that she's leaving).

If I were to hit someone in a heated argument, then yes, I would instantly regret it. I would also not be at all surprised if the person I hit didn't want to have anything to do with me again and reported me to the police to be arrested for assault. It is not an acceptable way of dealing with frustration or anger. Walk off. Shout louder. Hit the furniture or a wall or yourself if you absolutely must hit something. An argument is not an acceptable excuse for violence. There was no threat of violence or injury in the argument, so no reason to start using it. Instantly regretting it isn't sufficient. If they instantly regret it and then sign up for anger management courses and voluntarily stay away from the person they've attacked until they've successfully finished those courses (and plead guilty and accept the punishment if charges *are* brought) then perhaps that is sufficient. It's also probably sufficiently rare that it's not worth worrying about in the figures.

I'm sorry, this feels like an attempt to say that a particular level of violence isn't actually violent or isn't actually illegal, or (similarly) a particular level of sexual coercion/force isn't actually rape.

but I'm not sure it tells us for definite anything we don't know already.

Possibly not, if what you already knew was that rape and domestic violence against women was a severe and under-reported problem where society continually fails the victims. Of course, if you were hopeful that since the last N surveys to give similar results something might have been *done* about the problem, it tells you that it hadn't.

and unfortunately society is, if anything, even more tolerant of domestic violence against men

Have you got any figures to back that up? Cases like these two make me think it's the other way round because much more tolerance would mean that it would be legal!
http://www.thisislondon.co.uk/news/article-23409387-details/Top%20doctor%20who%20hit%20his%20wife%2024%20times%20is%20spared%20jail/article.do
http://www.dailymail.co.uk/news/article-476644/Executive-branded-wife-iron-freed-2-000-fine.html