I personally don't think Big Mom is even on this ship.
1. Tamago and Pekoms arrived in Fishmen Island in preparations for a Tea Party of sorts. This party was in four days. It was never confirmed to be moved back two weeks as proposed by Tamago.
2. The overall time span from that instance to the current time of the arc is at most 3 Days. Punk Hazard didn't last more than a day, and it the current time in the arc is the day after Punk Hazard. Also, it likely doesn't take more than a day to leave FI. Otherwise how would Tamago and Pekoms even GET to BM in 4 days.
3. Thus, the Tea Party has to be at some relatively close time period. It makes no sense that Big Mom is sailing with the intent to hunt people down.
4. Pekoms and Tamago were last seen watching the Ceaser Display whilst in an unknown location. The location, however, hardly looked to be the same one as where Big Mom would be. More importantly, this is still within the party time which plays into the next point.
5. Why would BM be with Tamago and Pekoms? They were out collecting candy in preparations for the party. Why would they suddenly go back to BM and pick her up only to arrive somewhere very close to Dressrosa. The Yonko are prideful, there's no reason for her to suddenly be sailing with her task managers. Especially when she has a Tea Party planned, and the former two were recently in an underworld pub location.
This wouldn't be the first time someone's communicated with a Den Den Mushi that wasn't visible.... one example is Vergo. Also, it simply could have been Pekoms making a praising statement. Nothing indicates BM is on the ship, and the ship was only Identified as her property... not that this ship is her personal ship. It's actually more likely her floating Cake Island is her ship.
Anyways, they can escape without TOO much work. Without help, Brook and Sanji will have to face Tamago and Pekoms...
Sanji vs Tamago
Brook vs Pekoms
Now I don't believe they'll finish the fight. Ceaser will be unlocked to help face the enemy so he can deal with the "Fodder" so to speak. Chopper and Momo will prepare to use the Coupe de Burst. Nami will use her weather Tact and disable Big Mom's ship, and then they will retreat via Coup de Burst. It will be the first clash between the two forces.
I could see Kidd then arriving to "clean up."
Still, Kidd saving the day or Jinbei arriving is also a viable option.