It is definitely and interesting discussion.
I don’t know your clients skill set/experience or the nature of the forms, changes etc. I think it is likely you will need to engage in a conversation with them about the realities of tool making. Here is my 2cents worth.
If Solidworks feature tree is required, then so is Solidworks. Native file formats is a moving target at best of times. If you were using SolidEdge, NX, Inventor or ANY other system the problem would remain exactly the same. Which is precisely why an open NON history generic format is used for sharing shape geometry.
In reality ZW3D is a powerful integrated CADCAM system that makes mods to CAM connected geometry fast and efficient within one environment once it is at the tool makers. As far as I am aware even Solidworks integrated CAM translates between software to generic formats.
Once shape has been worked on, it should become the master shape and subsequent work be modification of that shape.
E.g, when you need to modify an existing tool, you would only ever modify the original shape used to generate the tool, not the parent model.
If they want to do the mods themselves, they should just get on with it. Then send you the correct geometry.
If they made the model in the first place, then they have the history.
If they now want to edit shapes you have edited, they can either buy a ZW3D License or start with the generic which is the right process anyway.
So skill set may be an issue their end???
Curious to hear other opinions.