I would suggest to plug the hard drive in the pi too. II’d say that’s the only way you will get a smooth experience watching movies on your pi. You asked how I solved my problem… I bought a 5.0Ghz WiFi dongle. The integrated WiFi often won’t be sufficient to handle HD streaming over WiFi, all the more in a crowded place (don’t know if it is your case).
As for your questions, OSMC be easily set as a samba server so you’d get exactly the same functions.
I’ve rapidly tried to connect the HD directly to the Pi and switched the smb app option on.
I had no access on the mac to the HD : i can see the share in the finder, but that’s all… An idea to tell you more about my problem ?