I’m no expert but I will try to help you with this one.
1. Some say that it can be kept in a cool room for up to 3 days. I like to refrigerate all my cakes. Especially since I like using perishable fillings.
2. Yes it can be used under fondant. I use it quite often.
3. It is not a crusting buttercream but does firm up when you refrigerate it. I always do that before covering it with fondant.
4. I prefer to make it fresh but you have frozen leftover. Just bring it to room temperature and rewhip.
5. Never tried it but you probably can.
6. https://fromscratchsf.wordpress.com/2011/03/08/tutorial-swiss-meringue-buttercream/ This is the way I make mine. Used to have pictures showing you all the stages. This particular recipe has less sugar than most which is great for my family. They don’t like the frosting too sweet. If I’m going to add something that is a little tart I will add a little more sugar. This has always worked for me.
Hope I was able to help a little.
