It was predictable. It is also seems predictable that Mo will bond with William and not want to give him up, maybe even to the point of contesting custody. After all, arguably, Sorcha's finace isn't a blood relative either so has no more claim to him. It will rather depend on how long they've been together and whether he has any parental responsibility legally.
No, chizzy wasn't pregnant when she joined. I think it was just done to add a little mystery to her character in the first few months as they didn't announce she was pregnant in the initial character biogs - it was something like, there is something huge with Mo which will be obvious the first time you see her. We then weren't told the circumstances until the episode she gave birth which was over 2 months later so there was uncertainty all that time - was she married, had she been dumped, would she be leaving quickly etc. It was just a 'hook' dramatically. It also had the added benefit of adding an extra dynamic to the jonny/Jac situation - as Marcel said at the time (an interview i reread yesterday as it happened) - Jac doesn't want anybody to know about her and Jonny, she doesn't understand his relationship with Mo, she can't help wondering if he is the father but can't ask as they aren't meant to be 'serious' and finally, she immediately discounts Mo as being any competition professionally as she thinks getting pregnant is the worst thing a female surgeon can do and assumes she'll be buggering off within a couple of months so isn't worthy of any respect or investment as a colleague.
I doibt they had Emma planned at that early stage, they'll have wanted to see how viewers responded to Jonny and his relationship with Jac aside from anything else. If you think about it, she fell pregnant almost exactly a year after they arrived so it will probably have been plotted at the second or third storylining conference after they arrived as it started in the second half of season 15 whereas the surrogacy was mid Season 14
Also, i think they did it to show what a kind person Mo is but then show her having 'lost' something which arguably made her slightly more guarded and bitter - especially when she was 'losing' her best friend to Jac around the same time. It was mentioned a few times that Mo didn't feel she was getting the support she'd expected from Jonny after the surrogacy because he was so wrapped up in his love life.
Finally, it did make a connection with Jac as the birth forced them to open up to each other a little in a way they wouldn't normally do.