I think we need some step-by-step debugging. No, I don't know why things are not working.
root@marietto:/home/marietto # mount -t auto /dev/da1p2 /mnt/da1p2
mount: /dev/da1p2: Operation not supported by device
The error indicates that you tried to do something to a device which the device is not capable of doing. Bizarre examples are things like asking a disk for "eject paper" (yes, such a SCSI command does exit!), or asking a printer to read a disk sector. Obviously, you didn't do these bizarre examples. But the indications here are that you are asking either /dev/da1p2 or /mnt/da1p2 to do something they are not capable of.
Does /dev/da1p2 exist, at this time? If yes, what kind of thing is it? If you do "ls -l" on it, it should have permissions that start with "crw-...", indicating that it is a character device, and you can do read and write on it (I'm assuming you are root when executing that command). If you do "file /dev/da1p2", it should say "character special". If you do "dd if=/dev/da1p2 of=/dev/null bs=1048576 count=1024", to read the first gigabyte of the disk, it should work fine. It's a little harder to check whether it is writeable, but if it weren't, you could try the same mount command with "-o ro", and it should start working. Note that for unusual devices (such as emulated disks in virtualization settings), the permission bits might say "writeable" (as the "crw-..." indicates), but an attempt to write might get either ENODEV (operation not supported by device) or EPERM or EROFS or pretty much any other crazy error number that the implementor of the virtualization layer came up with.
Does /mnt/da1p2 exist? Is it a directory? Is it writeable? Do a "ls -l" on it. Try to create and delete a file in it: "touch /mnt/da1p2/foo" followed by "rm /mnt/da1p2/foo". All that has to work.
I don't like using "-t auto" on the mount command; it forces mount to guess the file system. I would use either "-t ufs", or leave it off, since ufs is the default. But you did that in the next command, leading us to ...
root@marietto:/home/marietto # mount -t ufs /dev/da1p2 /mnt/da1p2
mount: /dev/da1p2: No such file or directory
Was this done at the same time as the command above? Recheck that both /dev/da1p2 and /mnt/da1p2 exist and are the correct type of thing (character device and directory, both writeable).
I assume the next line is from "gpart show /dev/da1":
...
264192 1953259520 2 dragonfly-label64 (931G)
The partition type says that /dev/da1p2 is not a UFS file system, but a DragonFly file system, perhaps Hammer. But that doesn't have to mean that this is what it contains: it is perfectly possible to put data for file system X into a partition that's labelled to be type Y (but a bad idea). You might want to check what data is really in there with "fstyp /dev/da1p2", but I don't know whether that's reliable in detecting DragonFly file systems. You can also try "file -s /dev/da1p2"; again, I know that it will reliably detect UFS when run on FreeBSD, but again, I would not trust it for foreign file systems.
For amusement, here's the output on my system (where the boot disk is known as ada1p2):
# fstyp /dev/ada1p2
ufs
# file -s /dev/ada1p2
/dev/ada0p2: Unix Fast File system [v2] (little-endian) last mounted on /, last written at Sun Dec 26 13:54:12 2021, clean flag 0, readonly flag 0, number of blocks 1572864, number of data blocks 1522519, number of cylinder groups 10, block size 32768, fragment size 4096, average file size 16384, average number of files in dir 64, pending blocks to free 0, pending inodes to free 0, system-wide uuid 0, minimum percentage of free blocks 8, TIME optimization
