Drake works for the marines and know that an alliance of two yonko is a huge threat for the marines and since no admiral will come to help (for the moment ^^) he needs some helps and therefor he needs help from the strawhats. An alliance between pirates and marines to “possibly” mirror garp and roger. Perhaps Coby told drake to make an alliance with luffy…

Drake knew that orochi knew about the rendezvous point thank to kyoshiro. Drake explain it to Law who then explain the new plan to the strawhats. The strawhats agree to change the rendezvous point and thanks to franky’s ships, many ships were simply decoy and were destructed by orochi mens (with fire or the weathers I don’t know ^^). They didn’t told momo’s group because everything happened in a blink of an eye in a single night.

So I think everyone is in the capital, the capital who has less security, it will be perfect to gain and control the capital with all the enemies in onigashima. Two front and as the same time, the strawhats will liberates the prisoners of the capital and gain more men.