If modifications are done on Out of Box Schemas, what is the engineering stand on such customizations, and will R&D support future issues related to these modified OOTB Schemas/workflows?
The out-of-box schemas and workflows must not be modified. (Schema can be extended using schema extension mechanism).
The customizations need to be modified to respect that rule before being deployed in production.
Those modifications are dangerous, and the customer will have functional and upgrade troubles (conflict and deletion of a part of the customization).
Engineering is not supporting any issues coming from out-of-box schemas and workflow modifications.