I managed to get through all three of the early courses and now I am going through the new material. I notice a difference in the structure around “having something”, like having children, for instance.
In the first course it was: Oyce plant gidatee? Oyce, my plant gidavee (I have is around children)
In this course, it is: Oyce gidatee plant? Oyce, My gidavee plant. ( I have is before children)
Am I safe in understanding that either is correct?
You are very safe 
Diolch!
Also for the negative? No I don not have children:
Nagoyce, doyce dim gidavee plant OR Nagoyce, doyce dim plant gidavee
Safe as houses! Although, does ddim plant gyda fi is what I would say.
So is there any difference with the ‘Northern’ forms:
Oes gent ti plant
Oes plant gent ti?
Technically, you’d be ‘oes gen ti blant?’ and ‘oes plant gen ti?’… 
Aha ‘gen ti’ ! You know, sometimes it is really useful to have these little things pointed out / confirmed, especially things that don’t effect pronunciation!