In SF4/5, there’s an SPD shortcut. You technically only need to do a half circle input + one more direction, so they’re much easier to do with the shortcut. In SF2, and in most all SF games prior to SF4 era, you need to do at MINIMUm at 3/4 circle motion. It’s difficult to do in ST since you it’s also a much faster game, so it requires a bit of practice to get the technique down. For me, it helps to only hit the cardinal directions and end in an upforward motion.
There’s several SPDs to do it especially if you apply more advanced methods, but the most basic way I do it is to do start in forward, down, back, up, up+forward + punch. I force myself to do it in up-forward since I have a tendency to press buttons early before I finish left hand motions.
Landing jumping attacks work differently in ST. There’s FAR less hit stun on jump attacks than what you’re used to, so what’s most likely happening is that you’re hitting them too high, and as you land, they’re throwing you as you attempt to finish your combo that won’t connect. A way to increase your chance of counting this is to either hit your jump attack late, or simply do an SPD as you land and throw them instead of them throwing you.
And yes, it is a legit defensive tactic depending on the range and spacing and character matchup. They’re called sac throws. You can find a lot of good, solid explanations over at the SRK ST Wiki. Easiest to just google it. Great for learning chars too and seeing their frame data and hitboxes.
And there are no stupid questions. Feel free to ask anything about the game you don’t understand or want more info about. Happy to help.