I'm not sure which MDG you are implementing?
I will answer based on MDG-Material, but the principle will still be the same for other MDG implementations.
1) Before you even get to the workflow part of the process, on pressing an action button the system will check the data as standard....it will check based on your ECC config but you can also develop custom check rules using either BRF+ OR code them using the BADI USMDZ7_RULE_SERVICE
2) Again, derivation of fields will also be done, not as part of workflow but, using BRF+ OR code them using the BADI USMDZ7_RULE_SERVICE.
Whether using the BADI or BRF+ you can state when you want the rules and derivations to take place. This is usually done based on CR type and step number. But you can set it for whatever condition you want.