Can an aircraft under owner maintenance be used as an instructing aircraft? Can an aircraft under owner maintenance be leased to a third party for use as a trainer if its for other than PPL?

You might have a look here in Standard 507.03(6) it says;

6) Special Certificate of Airworthiness - Owner-maintenance
(amended 2002/03/01; no previous version)

(a) A Special C of A in the owner-maintenance classification is issued for recreational purposes only.
(amended 2002/03/01; no previous version)
There might be a staff instruction or an AC out there that adds more to this but I think this is pretty clear.

Regards

Aircraft used for flight instruction need to be Commercially Registered with a Flight Training Unit and maintained by an appropriately rated AMO.

The only time a Privately Registered aircraft can be used for flight training is if the registered owner is receiving instruction from an FTU or freelance (Class 2 or higher) instructor.

Yes but,

http://www.tc.gc.ca/eng/civilaviation/r ... htm#406_03

A person who does not hold a flight training unit operator certificate may operate a flight training service if

the trainee is

(iii) using an aircraft that has been obtained from a person who is at arm’s length from the flight instructor, and the training is other than toward obtaining a pilot permit - recreational or a private pilot licence.