On he bottom area of toads tool where the stars are, shift + click the row of stars. That object will now appear in every act. If you only want it to appear in certain acts, select the individual stars without shift + clicking them.
I've been hacking sm64 since somewhere around 2012-13. I'm fairly experienced in modelling, and can do some basic hex editing. I have experience with most sm64 tools, though I am not very experienced in music making / porting and ASM related things. My strongest ability is modelling levels / objects. I'm pretty active around the site, and can usually be on the site a few times a day.
Ice slides are the bane of my existence(never done one before) but this is actually looking good but that floor tile... there's something off about it, it doesn't fit with the palette of the other textures. I'm sure it'll look much better if you change it to something else or color it to match with other tiles. For slides, be sure to put some design, not mario slides slides, hurh hurh and gets to the star, look at the cool cool mountain slide for example, or tall tall mountain, you actually have to control mario and sometimes jump.
I thought the same and I worked on the texturing a bit for him, fixed it up to be less eye splitting.
Pretty sure those rom hacks were made with an older version of the level importer. It used to expand to 48mb instead of 64. So you need to patch the ppf to a 48mb rom instead. Use an older version of the importer, linked below. Should work.