if you think the cranks are flexing and a new one ( which is cheaper than the original ) will help ... go for it ... Its a waste of money but whatever floats your boat ....
You have frame flex like in every folding bike ( and those so called regular bikes as well )
Mariners came in different ways over the years..the first ones were basically steel boardwalks, than crmo framed boardwalks, after that the Alloy frame which is currently used ....

No matter what .... foldies are best ridden when spinning in a higher cadence instead of mushed with low cadence. Maybe raise the seat a little and try to spin and the flexing will be a thing of the past ...

and free of charge

best Thor