All responses are assuming you are using OMV 4.x...
is it possible to build a newer version of Virtualbox?
Yep but stretch-backports has 5.1.30 in it (5.1.26 is the last I built). So, I didn't think I needed to keep building them. 5.2.4 is only 3 builds newer than 5.1.30.
PhpVirtualbox is also getting updated to work with 5.2 in the Github repository:
But no releases. So, that is why we aren't using it. I don't want to deal with bleeding edge releases especially there are new developers making a lot of changes.
the plugin has some serious problems if VM's are stopped and started for a few times leading to crashes aborting all VboxHeadless processes
This is a problem with virtualbox itself not the plugin.
I did not have these crashes with the official Virtualbox packages for Debian in the past.
It is the same code. Debian just splits the code into multiple packages. And maybe it is a new bug introduced in the code?? Over the years, I have seen plenty of issues with the official packages too.
Now that Debian abandoned Virtualbox anyway
What makes you think that? 5.2.4 is in buster and sid - link. 5.1.30 being available in backports is pretty good too.
wouldn't it be better to use the official ones again?
Nope. The virtualbox repo has issues all the time. I can't put their packages in bintray's repos due to licensing. Their package includes everything and is no need to install the QT gui client on a headless system. The official packages have different names which means the plugin dependencies have to be updated with every minor release. And finally, for some reason the official packages are horrible to upgrade between major versions.