It does shorten it because it wouldn't be a protracted argument. There wouldn't be a slew of conflicting opinions, because there is only one reasonable outcome - if the water is collected and diverted from the neighbours property, then the op hasn't contributed to the damage or acted in a negligent way. End of argument. I don't think any profesional would risk their reputation to provide a report that contradicts that. 😉
If you need help tying your shoelaces in the morning, I'll be busy, sorry.
You are absolutely incorrect here, and this is not a matter of opinion. The natural ground topogrophy should not be changed/altered, and overland flows dont have to be redirected at the side of OPs house. A pit in his backyard would be more than sufficient for the majority of the. It is not his problem at all regardless of whether the neighbour has cut or not - the water would still go that way anyway. There is no event where liability in the future will fall on OP, and there a professional would only be risking their reputation if they provided a report with your absurd argument/theory.