If you don't do an optional for one of the ambassadors, you won't be able to get the respective ambassador to side with you in the conference without a high diplomacy rank.
I did all the optionals the first time the dipo options were available but one was grayed out, im only envoy dipo right now so that must be why. The next time through I chose the i dont want to options, the dipo options were not available. Somi think i have it down. The dipo options are dependent on wether or not please these guys and on how much dipo exp you have.
Doesn't apply directly to the question, but I haven noticed the Kanaar optional isn't always the same. The first time I eventually got it from the Bajoran temple. Today, I went straight there to save time, and they got offended and told me to leave. I eventually got some from Ambassador Surah instead.
I think it randomly changes - I noticed on the Holosuite - it gave me the first response I got last time with a different person; I too tried going to the Bajoran Temple and was told off.