What’s the use dirsync option for mount

First and foremost, this is going to depend solely on your architecture, and customs.

I for instance mount things like this under /mnt. I know people that create top level directories, and people who put this stuff in /home. It all depends on what you're comfortable with. There is no distinct standard on this anymore, the architecture of the system has changed, and you have varying views now, on things that used to be 'gospel'. Things like /usr/local or /opt/share, rpm or source...you get the drift.

Secondly, if you re-read through your link at pathname.com, you'll notice the paragraph under /media that states

Rationale

Historically there have been a number of other different places used to mount removeable media such as /cdrom, /mnt or /mnt/cdrom. Placing the mount points for all removeable media directly in the root directory would potentially result in a large number of extra directories in /. Although the use of subdirectories in /mnt as a mount point has recently been common, it conflicts with a much older tradition of using /mnt directly as a temporary mount point.

So personally, I advocate /mnt/windows or some iteration of that. It keeps the top level dir free, and is simple and intuitive. When I'm looking through or auditing a system, that's where I look for mounts right off the bat.

The problem is that your /etc/mtab is not a file but a symlink to /proc/mounts. This has advantages but also the disadvantage that user does not work. You already guessed right the reason for that: "the system is confused when remembering who mounted the file system". This information is written to mtab, cannot be written there in your case though. The kernel doesn't care (doesn't even know) about user mounts (this is a userspace feature). Thus this info is not contained in /proc/mounts.

Do this:

cd /etc
cp mtab mtab.file
rm mtab
mv mtab.file mtab

umount as user should work after you have mounted the volume again.