So as far as you know, Jetpack shouldn’t interfere with any other opt-in or contact forms on the site (Mail-Chimp/WP forms, etc?)
No, it shouldn’t. Lots of our users also work with MailChimp and other plugins.
Do you know if I’ll lose all of my existing comments once activating Jetpack comments?
What system do you use for your comments right now? I don’t think you would lose them but it may depend on the plugin you used and how it behaves after you disable it.
how exactly would followers get notified of new content via this method? An email that says “site name” has posted a new article/blog” etc, with a link to new content?
Exactly ?? When you publish a new post, the subscription email includes a “by [author]” section below the post title. This uses the public display name settings from your site, which defaults to your username if you haven’t entered your real name. To change this, go to Users → My Profile and enter a first and last name, then you can change the Display name publicly as setting, to display a variation of your name instead of your username.
You can also choose if you want your subscribers to receive a full post or just the excerpt. In order to only have a short excerpt show in the subscriber emails go to Settings → Reading in your dashboard, and locate the option for “For each article in a feed, show” and make sure that’s set to Summary.
With the “Full text” option enabled, your followers will get the full content of your posts.