Hi mintleaf, welcome to the forum!
Just a quick thing, you can edit posts after you've sent them by clicking the 'modify' button in the top right of the post frame if you need to add another question or more information. Double-posting generally isn't encouraged so editing your posts is a better idea.
Giving and taking away powerups can be done as part of a shift using flags - it's actually the easiest way to use flags. I can't remember exactly how it looks in the normal editor so you may have to figure this out a bit, but when setting up a shift on a given screen, check out the 'flag off' and 'flag on' dropdowns. After all the numbered flags you should find a list of the power-ups. Setting one to 'flag on' will give the player that power when the shift is triggered, and setting one to 'flag off' will remove the power.
Just as a warning, this is slightly broken with regards to the umbrella and the hologram. You cannot remove the hologram using flags and if the player has the umbrella open when you remove it, it remains stuck open and can't be closed. You can overcome the umbrella glitch by putting a cutscene in the shift that removes it. I'm not sure if that works with the hologram though. Alternatively, if you use
egomassive's mod for Knytt Stories, KS+, it removes both of these glitches.
I'd actually recommend using KS+ anyway, as it provides a much more user-friendly editor (as well as some really awesome extra features).