Since my initial post, I've tried reformatting the HDD from NTFS to ExFAT. Both times it was a single one-word partition name with GUID Partition Table (versus MBR). I no longer seem to have issues with multiple simultaneous users, perhaps a router reboot solved that somewhere along the way.
What now seems to be a source of flakiness is that every time I transfer a file from my Mac to the Almond's SMB share, the Finder gives me this error at the end of any one file and stops transferring all subsequent files I had queued up for that same transfer:
Using "cp" through the Terminal allows me to copy files recursively, but it spits out an error after each file copy:
cp: /Volumes/Sojourner/[snip]: fchmod failed: Operation not supported
Also, I'm topping out at 11 megabytes per second uploading from my computer's internal HDD through gigabit ethernet to the Almond+. The drive connected to the Almond+ is a brand new USB 3.0 WD MyBook 3TB. When I'm pulling data down from the Almond+ along the same path I'm seeing about 31 megabytes per second. Both are sufficiently fast for my needs, but just wanted to give you some real-world feedback.
The load on the Almond+, reported through OpenWRT, during one upload, and watching two movies on separate machines all at the same time was between 1 and 2.