"Send Test" buttons greyed out inactive
There shouldn't be any connection between the "Opt-Out"/"Unsubscribe" token and the field "Responses: Opt-Out Message". Both are required.
If the "Opt-Out" token (or "Unsubscribe" token) is missing, then you should get a very loud error message. (The token is generally provided as part of a header, footer, or message template, so that you don't have to put it in explicitly.)
The field "Responses: Opt-Out Message" should be filled in by default. If it's not, then perhaps someone removed the default. Try navigating to "Mailings => Headers, Footers, and Automated Messages". Edit one of the "OptOut" messages and set it as default.