-How can I block all UDP traffic going to the router from the outside, but only allow from a “list of IPs” and MAC addresses?
I also have a second part to this question.
-If “Device A” goes through “Client’s Router” > Internet > Mikrotik Router. By the time it reaches the Mikrotik, does it carry the MAC Address from “Device A” or the “Client’s Router”?
Thank you in Advance! 
-How can I block all UDP traffic going to the router from the outside, but only allow from a “list of IPs” and MAC addresses?
I also have a second part to this question.
Use address-list to block list of IP address.
There is no option to make the same list for MAC-addresses.
-If “Device A” goes through “Client’s Router” > Internet > Mikrotik Router. By the time it reaches the Mikrotik, does it carry the MAC Address from “Device A” or the “Client’s Router”?
No, MikroTik receives packets with MAC src-address, which is taken from MikroTik router gateway.