Hi allan,
I use sub apps but I’m moving away from it.
Here are the answers for the questions in 2017 
- Does a sub-app have all the functionality of an app including domain, SEO & metatags, sendgrid key etc. ?
yes - as long as you pay for an additional plan. My main app is on the team plan (legacy) and the sub apps are all on the personal plan (legacy). This means that you are limited if you want to work on certain workflows which are only available in professional plans and above.
- Is the database of a sub-app kept untouched when deploying from main-app ?
yes, it’s completely separate. the only thing that it’s copied over are the options sets.
- Can each sub-app use a different Stripe subscription plan ? (I need to make the Stripe subscription price variable per sub-app)
Not sure about this one but as along as the sub plan is dynamic you can change it on each app.
- Can each sub-app use a different google analytics account ?
yes
- Can each sub-app use a different mailchimp account ?
yes, as long as you make the mailchimp account dynamic. You need to fetch the data from a field in the database.