Welcome. Since you mentioned Charazay, I'd like to point out that I have read that game's user manual before when deciding whether or not to sign up for that game, and noticed two big things that I believe give BB the edge; season length (17 weeks is too long imo. 14 weeks is just right), and specific player skills (outside scoring/defence and inside scoring/defence require two completely different skillsets in BB). But anyways, to answer the questions;
1: Your starting crop in BB can be useful, especially since 'youth' players aren't readily available like they are in other manager games. Whilst it's still a good idea to fire the crappier ones almost immediately (unless you want to keep them as scrimmage fodder), some players in the initial crop, especially young players with good potential like Antuma, can last you several seasons. As for economy, it's driven mainly by weekly player wage, making the best bigmen in the world very cheap to buy, but very expensive to keep, whilst the opposite is true for a well-rounded small forward.
2: Yes, training in this game revolves around focusing on a select group of prospects, whilst ignoring the rest. However, you only need to play a trainee for 48 minutes TOTAL each week in the position you are training them in. Hence, it's not that hard to keep them close to the ideal 60 minutes playing time each week to keep their game shape high whilst training...
3: Keep a 12 man roster, and play a scrimmage each week. Scrimmages give your reserves a chance to catch up on the minutes they need to maintain their game shape, and with a 12 man roster the math works out perfectly for each player getting an average of 60 minutes for the week, which is ideal. And whilst salary is usually a good indicator of who the better player is, it shouldn't always be relied on, especially once you start training players. Instead get familiar with the actual player skills to make a more educated decision on who gets more playing time in your league games.
4: Arena expansions, especially when you still have the initial 5,000 seats, are a good investment. You stand to make all the money you spend back within two to three seasons, and from there it's all profit. Try not to over-expand though.
5: A good doctor will reduce the chances of your player being injured for long periods of time when they do get injured. That said, injuries aren't that common anyway, and for the sake of your finances I can only recommend getting a Basic doctor with low wage, and the massage speciality if you can find one. Same goes for your PR manager; the Crowd Involvement and National Appeal specialities both give your team a bit of an extra edge in games so it might be worth getting one. The trainer is the only staff member type that I recommend hiring anyone better than Basic in right now, but only if you want Antuma to train faster. This might not necessarily be a priority for you...
6: Same nation players will earn you more merchandise than equivalent foreigners, but I believe that an U21/senior NT players generates a merchandising bonus too, even if he's playing outside his home country. That said, NT players of any country are probably outside your price range this early on.
7: It's tied in to your TV contract. In division III, the weekly floor is exactly 100% of your weekly TV contract. The % is higher in higher divisions, and lower in lower divisions.
8: Yes, for the most part, only the champions of each league promote to the next division up at the end of the season. However, sometimes automatic bot demotions result in a leftover spot/s in the league pyramid, and if this happens, these spots are filled by the team/s with the best regular season record across the entire division, who did not become champions of their respective league.
Reached the character limit, I'll answer the remaining questions in the following post...