JohanFourie

I stand corrected: I've tested this all day and you're 100% spot-on, the Power Limit is not an Inverter Limit but rather just a Battery Limit - which is even more perfect for OP's purposes. I've subsequently set my own limits during daytime as well; my battery sends its thanks to you for probably extending its life!
 
Yes exactly: I was referring to the “Discharge Amps” setting on the "Battery Type" screen which applies across the board (Grid Up + Grid Down) unlike Sytem Mode-System1 which is Grid-Up only if I'm not mistaken;
 
 
Yeah it's perfect for your scenario; and as @ZaydS mentions, there's no harm in having it specified throughout the day since it doesn't impact PV/solar production.
 
 
 

Yes. Search this forum for "ICC"; that runs on a Raspberry Pi and talks to the inverter. You can just click on a button to switch to/from line mode. In conjunction with a BMV battery monitor, you can switch the inverter between utility and battery at a precise SOC percentage.   You download the appropriate file to your Windows computer, then use a USB to serial converter and the provided RS-232 to RJ-45 cable to talk to the inverter from your computer. The firmware update file is a .zip or .rar file, and inside is a special program for firmware updating. You run that program, and about 8 minutes later, the new firmware is running.
See Can I update my firmware, and if so to what?
I don't use it myself, but I don't think it can do both. But you just rearrange the cables for the firmware update, and when it's done, restore the cables for monitoring.