>develop black powder through trial and error for basic shrapnel centered explosives
Again, one of the limiting factors is metallurgy. You have to be capable of making spherical metallic objects, which is a challenge to do from nothing. You either need tons of money, connections, or just straight up magic to do this. Plus, people will steal the crap out of your idea.
Also, grenades are really dangerous, especially early grenades. 18th century armies had dedicated grenadiers to handle them. You stand a good chance of blowing yourself up.
>figure out fertilizer through bird shit
Nope. Manure was used as fertilizer as early as eight thousand years ago. It's not very new.
>apply basic physics knowledge to magic to make it more efficient
Sure. Dependent on the setting though. However, note that most people don't necessary understand physics as much as they think, they just know physics concepts from our standard model.
>fuck nippon cooking introduce the people to mustard and basic sugars
There's a reason that sugars and spices were a luxury during medieval times, they require specific environments to grow. Furthermore, this is completely dependent on the flora and fauna of the area. There's not likely to be much low-hanging fruit for you to pick here.
How do you spread propaganda quickly to people who can't read? Peasant rebellions happened all the time, and they are all put down relatively easily.
Again, the problem is initial resources, if the person starts out with a lot they can make it big. Otherwise, it'll be rough.
The effectiveness of this, like most things, is dependent on the amount of time that needs to go into it and if you can actually make it out of raw materials found in nature. It's one thing to be able to obtain materials from a store, it's another to harvest it from people's shit.