1.) That infinity sign is actually a "chain-link" sign. I don't know why it won't crossfade but my guess is that merging the tracks is different from bouncing in a sense that merging is non-destructive. Instead of merging, try selecting your events/clips and bounce them. That way you can still cross-fade.
2.) Cut the part that you want to elongate. Open up the inspector (F4) and you can see which timestretch modes you want. I suggest to use Solo - Elastique Pro Monophonic Formant. Hold "Alt" key and drag the end of the event to stretch it.
3.) When I'm hiding tracks, I disable them first. Right click on the track header and select "Disable Track" before hiding.
4.) Save As does not create a copy of the directory. It just points to the same folder. If you want to create an entire copy of the folder, select Save to New Folder. Then one of the easiest ways to save space is by going to Song > Remove Unused Files. You can also clean up the cache but I don't recommend it. Studio One will just recache everything and you're going to have a slow session. To clean the cache up simply go to View > Performance Monitor then beside the "Cache" word you'll see a wrench icon. Click that and you'll see an option to clean the cache.