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Your first question:
Although without a heatsink H6 boards easily go up to 70°C (still 10° behind throttling threshold), If you either install a fan or underclock the CPU to around 1.4 GHz, you can go 24/7 assuming there is no power outage or SW issue.
 
Second:
As you have already had a somehow similar server, you might know some issues that might happen, for example if you use microsdcard as your storage device, you won't get a super reliable server. (but I think using emmc might solve the problem)
Another thing I noticed is that when I installed pihole, the lighttpd stopped working after sometime randomly (it only hurt the web ui, the dns service was still running) , and the problem didn't go when I replaced it with apache, even after I completely removed FTL from pihole, and installed WP for other stuff. This is somehow strange because for example this had never occurred to my BLYNK server or cockpit on my OPi lite 2 board.
 
Third:
If you mean the OPi3 board, I'm not sure about its USB hub status, but I have OPi Lite 2 and the USB 3 works just fine. I haven't tested the speed yet, and I don't know whether it is behaving just like USB 2 or is in SS mode but I assume it is running at SS mode, because the output of lsusb command recognizes it as USB 3.
 
Summary:
If you install proper heatsink and cooling fan you can run the server 24/7 as I have had an uptime of around three weeks with OPi lite 2, and I haven't experienced any serious issues, and issues I said above can differ case to case. I think the board can be a suitable replacement/upgrade over OPi zero