I like mysql.mydomain.tld because it's obvious what I'm using the host for and it's very unlikely that I will need to use the subdomain for anything else.
No and no.
I don't think there's a strong preference one way or the other and since you can have both on shared hosting, there's not a big impact when an app supports both. I tend to like 5.x over 4.x.
One thing is that when you have a dedicated or PS host, you'll only have one default so it's easier if you pick the one that matches the majority of the apps you're going to use.
You're very welcome!
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