2) Music production takes time, years in fact. Over this time you're losing a lot of money by "renting" Serum. Save up and buy it imo.

Nope, you get to keep it after you've paid as much as the full license is.

This sweeping technique is very useful on drums, specially rides. There can be a lot of ringing/whistling freqs in a ridehat. A good rule of thumb is to cut a lot on the very annoying frequencies and cut less on the less annoying frequencies. As said before though, in some sounds you can find a lot of ringing frequencies and if you take them all out you will ruin the sound. It's important to find a good balance and to A/B during the process.

I disagree with compression not being a cleaning tool. It depends however we define "cleaning up". With compression you smooth things out and have elements pop out of the mix resulting in a better defined mixdown and less messy. In the end it's all about several smart decisions made to get a great clean mix.

Settings really depends on the sound, you have to experiment with the settings till you get the best one. As mentioned above, compression is one one of the most misused tool. Always make sure the input level matches the output level on the compressor. Once you have your settings, bypass it on and off to see if it makes things better. If things sounds worse, redo or completely remove.

Compression can smooth things out and make it sound bigger. But I think one of the best tricks is to add white noise. It will go a long way!