1) 1,500 watts on a 12 volt system is about 150 amps. How long will the wire run be? I would not go smaller than about 2/0 at that current. Windy Nation rates their 2/0 at 325 amps, but that is 105C temp rated, the NEC lists 75C 2/0 cable at 175 amps. But you also need to take the length in account. Losses at 12 volts add up fast. Lose 1.2 volt, and that is 10%.
2) I do not know that Victron unit. Are the voltage settings able to be dialed down for 12 volt? 15 amps will charge slow, as that is under 200 watts, but it will charge.
3) I am happy using the app in my JK BMS, the Overkill looks similar.
4) Setting the charge controller to 3.4 volts per cell on LFP will get you about 95% charged. The discharge side is a bit tougher. LFP cells near 50% do not change voltage much at all. Even after doing a full cycle to know your full capacity, you would need to watch the battery SOC to see when you get close to 50%. Most inverters etc. only go by voltage, and with LFP, you wont se a drop in voltage until down more like to 20%. Are you doing this for the storage of the cells? You could run a heater on a timer, once you know what your capacity is. Dial it in from the SOC on the BMS.