Yes. This is the reason why SFs are hard to train. Not because it's difficult conceptually, but rather because it's tricky to find a way to win league games while still training guys out of position.
Still, the rarity of SFs means that if you can train your own, you'll have a mismatch and your team can gain a nice competitive advantage over other clubs with the same total salary space.
Very true. If I were you, I wqould buy some 7k complimentary playeers, and scrubs and stay in D5 for 6+ seasons. You can build up your arena, gain cash to buy better players, and eventually promote. Train your SF now, as you might not be able to do so in D4.
I wouldn't suggest intentionally staying down. Training guys out of position can be a challenge but it's not one that requires one to stay down and avoid competition altogether. Sometimes the experience you get trying to work your tactics around a known deficiency in your lineup can be great experience for trying to set tactics against teams with a player or two that simply outclass anyone on your roster.
Besides, if it were impossible to train players out of position in IV, I'd just recommend promoting to III since it seems to work fine here. (It's not easy by any stretch of the imagination, of course, but it can be done!)