Is the driver’s door lock on an R50/53 mechanically actuated/operated?
If the battery were to die, or if the electronics failed, would the key still open a locked driver door?

 

Yes

 

As an aside, if your battery fails you can also still open the rear hatch via a cable under the rear seat.

 

Cars before about October 2002 do not have this feature - a retrofit kit is/was available.

 

Any chance you could point me to this kit?

Is it 51247119986?

Thanks.

 

Maybe!
I'd check with your friendly MINI $$$ealer. I'd heard that because so few early cars need it there may be none left.
It's only a cable attached to the catch & I believe others have made their own. It doesn't look that difficult but I haven't tried.