What do I do with unused spackle

You can use either spackle or joint compound. I personally detest the modern "light weight" versions of spackle and will not use them. I use joint compound for most repair applications around drywall. The dry time has never been a big problem for me as most projects are big enough that there are plenty of other tasks to attend to whilst the joint compound dries.

Note: Most of the preferred "old style" heavy spackle is also a pain to use in comparison to joint compound because it drys really hard and is a total pain to sand down. Joint compound is really easy to sand thus giving it one more feather in its cap.

1. Remove high spots by hand sanding the entire wall with a flat, rigid 1/2-sheet sanding block and 80 grit paper. It's ok to remove too much, but stop wherever you start to see the sheet rock facing paper or joint tape. Ideally, 100% of the wall will have been sanded.

2. For the areas (if any) where the paper shows before all the ridges (as in your photo) are gone, switch to either a 1/3-sheet or 1/4-sheet flat and rigid sanding block with 100 grit and sand broadly over the areas surrounding the show-throughs.

3. Remove the dust by wiping down the wall with a damp cloth/sponge kept clean by frequent rinsing in a large bucket.

4. Apply a thin coat of spackle using a 10" or larger metal blade. Apply less than required.

5. When dry, shave off any specs etc using the 10" blade over the entire wall.

6. Use a damp 4"x6"x2" sponge to 'sand' the new spackle, working the sponge from the middle of a patch of spackle outwards towards the edge and beyond.

7. Apply a second thin coat of spackle and repeat the shave/sponge steps.

8. Inspect for flaws using a bright flashlight or trouble-light held against the wall with the room lights turned off.

9. Touch up as necessary.