No red questions to answer,but.

"connected to, from your main pc". There should be a comma to break up that statement indicate that the main PC will be connecting to the slave PCs. So, yea, Inbound firewall rules on your slave PCs. You should also create the same inbound firewall rules on your main PC, or specifically the ISBoxer TCP: 10102, for the multi computer helper, if the helper is not detecting the other PCs.

Game path errors usually arise when the Game Profiles you select on your main PC when configuring characters are only the game profiles local to your main PC. So if they are named differently on the slave PC, then the slave PC wont be able to find the game profile. Paths usually only come into it when you have multiple game profiles on the main PC, which match by name multiple game profiles on a slave PC, and one of them doesn't work.
You need to go through each character in the top left, and make sure the Game/Game Profile dropdowns match appropriate values for the PC they will be running on. This might mean syncing it up so both PC's have the same names for the Game/Game Profiles (although they can have different installation paths), although it does sound like you have the right options to select for Game/Game Profile, just that toon nr 2, is not using the same as toon 3 to 10.