1. Training. Your training example sounds really slow to me. Did he have full 48 min week in week out and what's his potential? These are the only reasons I can think of. By the way, you certainly don't need lv6 trainer at your level. They are for national team trainers or div I/II. lv 4 or 5 gives you more bang for you buck. I would get a 12k-15k salary lv 4 trainer (no special skill) if I were you.
2. Arena. Many players agree expanding arena up 20k pays out in the long term. Your arena is way too small. Start from the cheapest seat and then slowly add the more expensive ones when you move up. Unless you want to buy old/high salary players to storm your league and move up quickly. You should think long term.
3. Income. Try to win tv/rival/games before your home games and match your prices as you expand your arena. You will be rewarded if you get the ticket prices right. I think you are lucky that your team is born in US. You are blessed with a good choice of your countrymen. It should be very easy for you to maintain a roster of US players only and I am sure your merchandise will get a big boost. Even better if you keep your drafts and train them.
I think you should focus on training either guard/big men and buy those positions you do not train - they should have little training value, cheap and high salary (among your team). Then you should buy young trainees when the season starts. They are dirt cheap at that time. Try to get some bargains with good secondaries and potential that match you. If you do 2 position training, then you won't ever need pot 8 or higher. Don't waste your money on them. I personally recommend 1 position training with high potential. But don't ever pay big money for them. You are not there yet. Avoid those with high pot and 20 yo or higher. There is a very good chance you won't ever train them up to their pot. If they are well trained, you can't afford them yet. I got a pot 8 and 18 yo for 11k at season start. You might do the same. I would get 3 of them and get them trained 48 min every week. After getting the veterans and trainees, the rest of your money should go to your arena. There should be nowhere else to spend money and if you get these things right you should maintain a profit every week. Hope it helps.
1. Yes I usually get the 48 minutes per player as I scrimmage every week when not in the tournament. I'll look into a lower level trainer, was just hoping to get pops quicker with the more expensive trainers but it doesn't seem to do me any good.

2. Yep, I am starting to see the light on my stadium. Tooo small. I am going to start working on that.
3. I don't have a lot of USA players on my team. Usually I don't focus on that when bringing new players on. I always thought winning was more important but it looks like I need more yanks on my squad to boost merchandise sales. I don't know if I 100% agree with that but I'll work on it.
I usually do 2 Position training so that everyone on my team is pretty decent. Basically when my back-ups hit the court against my opponents my team will expand the lead or take the lead as they are similar to my starters and most teams back-ups are just that, back-ups. So far it has worked with playoff births in each of my seasons however it may be preventing me from taking that next leap as I don't have that one or two studs on my squad. Thanks for the advice on training, I'll look into it more.