A form submission that arrives by email arrives where email goes: in a pile, at the bottom, seen tomorrow.
With Slack connected, it arrives in the channel where the team already is, formatted so somebody can act on it without opening anything else.
The point is not the notification, it is the response time. A lead read within an hour converts differently from one read the next morning.
A bot token with chat:write is enough. It cannot read your conversations.
A Bot User OAuth Token (xoxb-), with chat:write and channels:read.
No. The rights above only let it post.
Yes. Connectors are part of the plan; there is no separate integration fee.
At any time, from the same panel. The credential is deleted at the broker.