Can I reduce the two-hour frequency to 30 minutes then?
Not that I'm aware of. You might be able to automate it with MC's various control interfaces (MCWS, etc) in some way, but I'm not sure.
Yes, you can, and there are instructions for it for Windows the Auto-Import wiki article:
http://wiki.jriver.com/index.php/Auto-ImportBrian, if you ever need to, this can easily be done on OSX too, I was just too lazy to write up instructions.
2) Regarding the import dates, it is probably a bug. I definitely see the albums on the top of my list I entered into the system around 3-4 years back. Is there a way to troubleshoot this somehow?
I'm fairly skeptical here, as we'd see people with crazy eyes crawling out of the woodwork. I'm certainly NOT seeing anything similar.
I'd recommend
disabling the Fix Broken Links feature of Auto-Import, and see if it goes away. The default Fix Broken Links feature tries to protect network volumes, but is not 100% foolproof. If files are being removed and then re-added (which can happen with some NAS devices, and I suspect the Venn diagram of those that don't support filesystem events and those that cause Fix Broken Links problems probably overlap somewhat).
In any case, if the following is occurring:
1. Files are imported from an external or network volume.
2. The files suddenly seem to "re-import" based on [Date Imported]
3. Other metadata remains intact
Those would be tell-tale signs of Auto-Import removing (because it thinks they're missing) and then re-importing tracks via the Fix Broken Links feature. It sounds like you don't need that feature anyway from your description of how you use the NAS. Just turn it off.