Passive components swapping and replacement
No. No.  

No.

Even in the best run company, purchasing need absolutely no encouragement to come up with ways to buy stuff other than what you have specified.

Each component you use should have a list of permissible suppliers. So for capacitors for instance, 'cooking grade' applications, in the case of MLCC ones you use for PSU decoupling and signal coupling, might be available from Murata, Kemet and about a dozen other suppliers, but their permissible part numbers must be specified. High performance applications, where you need low loss, or low inductance, will have a different part number and may only be available from one or a restricted few suppliers.

Obviously if you try to do this for every component you use at once, engineering staff will be overwhelmed. 

It would make sense therefore, for purchasing to report to you any difficulties they have sourcing components and have engineering staff pro-actively attempt to handle (say) 20 items a week.