I suspect that the ultimate escape of both Merle and Daryl is a plan by the Governor to implant Merle within the gates of the prison group. From there, he will be able wreak whatever havoc the governor has in plan..
The governor is playing Merle against his brother, claiming that Merle was responsible for letting the other group members into the compound but that's all part of his plan.
He knows that as long as the other group has the safety of the prison walls, he can't launch an attack on them and thus he has to have one of his group, Merle, embedded within those prison walls......
Yeah I’m not a zombie fan but I love the show with its post apocalyptic survival theme.
No. If you recall, Merle reported back to the Governor that he had killed Michonne on a previous mission. Imagine the Gov's surprise when he encounters Michonne in his house, quite alive and with a katana pointed at his (zombie) daughter's neck -- thus learning that Merle had been lying to him the whole time. The Governor is PISSED at Merle, which is why he's pinning the invasion on Merle.