This is interesting topic. Let's wait for BB to explain this in numbers (2nd part of your topic)
For the 1st part, it seems that previous ownership isn't considered in any calculations. I'm not saying it's correct that way, with relation to real world it doesn't make sense. Luka definitely has bigger sales in Lakers then some random Joe would if they brought him.
I had similar situation with one of my bigs I bought this season after couple of rounds... he had sales of 200$ even though his salary was over 100k until he played the required minimum of matches for my team. Now it jumped up a bit, but he isn't even close to my lther big with similar salary.
I assume, the bonus itself is pretty significant. My 19yo is leader in ast in league and is high in pts, he was in allstar and his merch is above 50% of his salary.