Ah, so often on the DIS when I talk about travel delays, I get a slew of folks who disagree with me. It is also most comforting to read the posts here, because I fully agree with most the posts here.
We've dined 1900PF a number of times, and been seated LATE a number of times. WDW stacks as many ADRs as possible. They are less concerned with seating guests on time than making $. (sad to say) Your ADR is fairly early- so your odds of being seated on time are better than guests who have a late(r) ADR time slot.
Still, more than once at GF we were seated an HOUR past our ADR, even though we arrived early. You probably won't wait THAT long- just know it is possible.
The timing of character meals is also a function of how the characters rotate, and how much you value seeing all of them. Typically, the characters come out- go around the tables - then take a break for a bit. Each dining location staggers the arrival of guests. Sometimes you see all the characters pretty quickly, other times we've been seated so that we are almost to dessert before they start coming to our table. You don't have to wait for all of them, but well, those character visits are built into the price of the meal. 1900 PF is among the most expensive buffets on property.
The third unknown is the timing on the monorails. They come every so many minutes. Sometimes you hop on one right away, other times you just miss 'em. Still other times- I daresay too often at WDW- the monorail breaks down (or has some other kind of delay).
Even if everything goes perfectly and you eat quickly -odds are very slim that you could get to Epcot by 10am. I'd say the 11 - 11:30 timing is more realistic, unless you have your own car and plan to drive to Epcot. If you plan to drive from GF to Epcot, then there's maybe a chance of getting to Epcot by 10:30am.
Oh, since it is that time of year- if your trip is over (or very near) the holidays: expect everything to take longer.