I dont think their relationship would have lasted if Jamie had survived. I think they would have got married, but Sonia would have tried to have found Rebecca, but Jamie wouldnt have been very understanding, and was jealous of the fact that it's a part of her life involving martin. They'd have argued on many things and realised that their marriage was over.
Sonia would have been disgraced by the Mitchell clan and then I think she would have got with Martin, having formed a bond during brining up Rebecca. I think the main problem in the Martin and Sonia relationship was that they married too young. They hadnt experienced life and knew nothing different.
It is clear since Rebecca has been on the scene that their relationship has grew stronger. When their marriage was on the rocks before it was because they wanted different things, when Rebecca came on the scene again the damage was already done, Sonia had already kissed Naomi. If they had got together whilst looking after Rebecca together I think their relationship would have been much different, as they would both have wanted the same thing: a calm family life and a stable home for Rebecca.