Excellent article Bhai Jasjit Singh jeeo. It was much needed. Many Gursikhs under the influence of Missionary Parchaar, do the uchaaran of ਮਹੱਲਾ as ਮਹਲਾ which is wrong. The correct and the traditional pronunciation is ਮਹੱਲਾ.
[www.gurugranthdarpan.com] - Professor Sahib Singh may be main proponent of the counter point and probably the main resource of Missionaries on pronunciation. His argument is presented in the link at the bottom.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥