Long story short:
Got a letter from my employer in June advising my promotion and salary review (substantial amount), to be effective from 1 July.
On 3 July, I turned in my resignation. I was then asked to extend my notice period to 6 weeks to close up projects.
I then found out that I will be paid based on my old salary till my end date.
My question is: should my employer honor the salary review and pay me accordance to the promotion during that 6 weeks period?
IMO - employment conditions/contract were varied by the letter, effective date was 1 July, so all pay after that date should be at the new rate. Resignation is a separate issue.
Unless there is something funny in your employment contract saying otherwise, I don't see how they can do that. Particularly in light of the additional notice period you are working.