Battlewarrior
1 For unwhored Qum it is already pretty much a solved thing, in Yhilin you should have 80k ProN to get started (assuming you did all the sidequests and chose the profitable options for the first Stineford section), get the mine for 25k, the bank for 25k, give megail 25k for her special opportunity supplier, get an extra 5k from the bank employee copy at the start of chapter 2 and then invest your remaining 10k on the magic shop in Stineford. Then during Megail's route you should receive 175k ProN, collect 10k from the Mine and 10k from the Bank, you get the Yhilini supplier for 10k, the Ardoheim magic trade route for 25k, the Eustrin Processor for 50k, the Embassy for 100k, and don't forget to pickup the bounty office for 20k Sx, you should have a leftover of 10k ProN. This gives you the best possible financial start for these first two rounds going into the Reunion section.
For whored Qum, it is questionable if you can get much out of the extra profits, but here is the most profitable start, in Yhilin you should have 95k ProN to get started, get the mine for 25k, the bank for 25k, premium steel for 20k, collect the 5k from the bank, and invest 10k in the Stineford magic shop, you should have 20k saved up. Then during Megail's route you should receive 160k ProN, collect 10k from the Mine and 10k from the Bank, you get the Ardoheim magic trade route for 25k, the Eustrin Processor for 50k, the Manufacturer for 25k, and the Embassy for 100k, don't forget to get the bounty office for 20k Sx. This gives you a stronger financial return during the Reunion section but in my opinion it kind of stalls there.
2 No.
3 Unless it ends up being a requirement for someone's final sidequest, probably not. There aren't many good investments left at that point in the game, but unless you need the 1 Erosia point for a threshold then you are probably better off spending it on something else.