I have several questions:
1. Does spinshot combo into anything?
2. What options can sonic not use without a double jump? Or vice versa, what options that sonic uses, uses up his double jump?
3. What moves does Sonic have that are safe on shield? (Or at very least -10 so cant be optimally punished)
4. What does ISDJ combo into
5. How many frames of invicibility does spin dash and spin charge have?
6. What the difference caused by the amount spin dash/charge is charged? Is there anything else than more speed and damage? Are new combos opened up by increased amount of charging?
7. What type of approaches do I have in neutral?
8. What are my go to combos off spin dash, spin charge?
9. What setups do I have for uair off the top kills?
1. Spinshot isn't an attack, and doesn't extend any combos so no. If you use it to mix them up you can create openings but there is nothing guaranteed.
2. Without a double jump you can't jump out of a spin dash or spin charge in the air, even if you land in the middle of the move. There's also BSBS, which you should check the guide for. It can get complex, but if you're worried about getting screwed over by it just jumping normally will make it go away.
3. Well spaced dtilt, ftilt and fsmash are somewhat safe. You might be able to cross up someone with nair and get away with it because of Sonic's mobility. Sonic is isn't really like Sheik in the sense that he wants to space moves, you're better off weaving in and out of your opponents range.
4. Pretty much anything you normally get off a spin dash.
5. Spin charge as no invincibility. Spin dash has a bit on the hop, which can be used to beat out a few projectiles like Mario's fireballs and counter attack.
6. The spin dash gets faster and stronger whilst I'm pretty sure spin charge just gets faster, in fact a slower spin charge allows you to get more multi hits in for more damage, so if you want an optimal spin combo a slow spin charge is the way to go.
7. Approaching is one of Sonic's weaker areas. As a bait and punish character he relies on your opponent leaving an opening for you to run in and punish. Cross uair and nair are ok but pale in comparison to something like a ZSS bair. In neutral your aim is to work out how the opponent reacts to the spin dash, then punish them accordingly e.g the opponent keeps shielding SD, so you SDSC and run in for a grab.
8. If you want an effective simple way of getting damage SD>SDJ>Nair is pretty good. You literally mash the A button when you hit with the SD and get about 22%. The most optimal spin dash combo is a slow SC>SDJ>Uair>Bair, but this only works on big characters or fastfallers at around low to mid%. A slightly easier variant is slow SC>SDJ>Uair>Nair which works on all but the floatiest of characters at low%, but the timing after the uair is strict.
SD>SDJ>Uair>Upb>Uair only true combos fastfallers, but if they airdodge immediately you can dair them. It can kill but it largely depends on stage selection, rage and staleness so you don't see it often.
There is some more complex stuff but these are effective bnbs.
9. SHFF uair>upb>uair is a true combo on the entire cast that can kill around 120% on average. Uthrow>upb>uair is a true combo that only kills if they're on a platform at around 50% (depends on stage selection). Its not a legit 50/50 since they should always jump away instead of airdodging. And there is also spring ride which isn't true but will kill if the opponent gets confused.