You can check whether a webhook has been sent for a new post, or for an edited post, by looking at the webhook’s X-Discourse-Event header. For a new post the header will be set to post_created. For an edited post it will be set to post_edited.
I haven’t used intergromat, but the process for filtering based on the event type on Zapier is outlined here: Discourse Webhooks and Zapier - #32 by simon.
Yes, I think so. I haven’t run into issues with the Post Event webhook, but I’d really like to see a way of limiting the User Event webhook so that it is only sent when a user is created or updated.