Yes, I’d been meaning to follow up on this. Guess things haven’t been going well for you so far. Sorry to hear that! Here are a few off-the-cuff suggestions for things you might try:
You say you’re getting flaky connectivity and interruptions. Is there any pattern to this? What type of connections and what are their speeds? I think that’s the first order of business to solve. If you could stick to Ethernet for everything, 1 Gbps or better for preference, at least whilst you’re troubleshooting, you might be able to identify a root cause more easily if you can determine exactly when your connection goes awry.
Then there’s the question of access privileges and seemingly disappearing objects. Before you look at applications and the Finder, can you use Terminal to reproduce the access issues you’re having? This is significant because Finder, especially in column view, makes a large number of requests to enumerate directory contents, and it would be easy to mistake such slowness for a permissions problem, especially if you miss the progress spinner in the status bar. So use Terminal to see if you can list, read and write files in a directory, create and delete files and directories, etc. Commands like
rmdir, and a bit of shell redirection should help you establish whether what you’re looking at is actually a permissions problem, or a problem of SMB more generally and inefficient Finder accesses through SMB in particular (which, FTR, really shouldn’t be exacting the sort of toll you’re paying on a Mac-to-Mac setup).
Finally the authentication failures. Especially with various versions of Windows in the mix, it is unfortunately the case that SMB is a bit of a hairy beast because various dialects/versions of the protocol support varying degrees of authentication and signing/encryption. Can you confirm that every Windows user who has an account has had their NTLMv2 credential stored? You do this by pressing the “Options …” button in the File Sharing details panel in Sharing settings, ticking the box for the user concerned, and typing in their password again. You only have to do this once. Then if you right-click each shared folder, open the advanced settings, and make sure mandatory signing and encryption are off. Does your situation now improve? Regrettably, ever since Apple moved to their own SMB implementation (from Samba, which Linux-based NAS boxes use), things have never been truly great for Windows, but they should at least be possible to configure to connect and expose the file share, with each user attaining his/her respective permissions.
Let us know what happens, and good luck. And sure, if it doesn’t go well, there are lots and lots of vendors selling boxes to make your life easier, all running real server operating system kernels.