Before the session, when it starts, and after it ends — with follow-ups that can go to everybody, only attendees, or only the people who did not make it.

Before, at start, and after the session.
Everybody, attendees only, or no-shows only.
Connect your SMTP so mail arrives as you.

Before the session, so it is in their calendar. At the start, so the people who forgot get a link at the moment it is useful. After the end, so the conversation continues. A webinar can carry as many of each as you need, in your words.
A follow-up can go to everybody, only to the people who attended, or only to the people who did not. The person who stayed to the end gets your offer again; the no-show gets the next session and a reason to come.
Connect your own SMTP and reminders are sent as you, from your domain, which is what lands them in the inbox rather than promotions. Each person gets each reminder once, however many times a background job retries.
For reminders on scheduled webinars, connect your SMTP so mail comes from your domain and lands in the inbox.
Yes. Write them in any language.
No — an on-demand session starts when somebody registers, so there is no "before". At-start and after-end reminders work in every mode.
Placeholders for the attendee’s name, the webinar, the session time and their personal join link.
Fourteen days with everything switched on, no card.