AY services sold as BA codeshares on 125 stock:
MMB: flights visible
Possible to see allocated seat: Yes (after you assign it on AY.com, using the same PNR, although you may have to use the Webchat function to get a seat assigned if it is not possible)
Possible to see seat plan: Yes, but if not, QR.com can make it visible
Possible to change seat online: Yes, on AY.com directly on the seat map, or via the Webchat function. If that fails, try QR.com
Online check- in on BA: Yes
Online boarding pass on BA: Yes: self printed, email and Save options
BA App boarding pass: Yes (not 100% I understand)
Booking visible on AY website: Yes, now using the same BA PNR (it used to be a separate PNR but they have now aligned this)
Seat management on AY website: Yes, see above
Online check-in on AY website: Yes