I cannot fully help but try to give you some advices.
I'm not sure. My pure guess is that you can switch facebook & mobile account in android version.
As far as I understand, any upgrade will give you additional seats. You will not waste your money. However, comparing with incremental upgrade, large upgrade should save some costs.
With 15% skill progress in whole match, he's not a good trainee. That's also reflected in his market price. In low levels, good trainee 's market price should be over 20 times his wages for young 18 or 19 years old players.