You can but your console will be banned and then you can't anymore. At a high level, online and CFW don't mix.
No, this will get you banned very quickly.
You can have a backup in emunand but not play online.
You can have a legit purchased game installed on your Sysnand and go online.
Not safe at all.
Latest update covers all previous updates. You only need the latest. DLC will be per title content. Each will be different and additive.
It will show you what's available to install in the shop you've connected to. If they don't have it you won't see it as an available update. There's another tool that will find what you're missing based on latest available but I will have to look for it.
Yes, tinfoil is safe to use.
What is a decryptor? Anyway, this is a huge question...can you be more specific rather than how does everything work? what are you looking to do?
You bet!