I’m not too clear on the problem, but it sounds like the laths are too short, no?
That is to say that the minimum width is determined by the motor (and weight of shutter) and not the opening. The guides and laths should fit this and would be oversize for the specified opening.
Get back on to the supplier with this and tell them that as they are the experts they should have pointed this out. You relied on their expert opinion and they gave bad advice. It shouldn’t require any more than a new set of laths.
That depends on whether I have understood correctly or not. Photos of the problem would be good.
It is a bit of a nightmare doing these things on line though.