Is credit history compulsory when applying mortgage ? Or is it a myth spread by credit card sales people ?
Yes all banks will do the checks as it is so easy to check with CTOS and CCRIS just online request.

- How much easier / faster can mortgage application be if I have a credit history ?
Good credit history will definitely help your loan get approved. Last year'a approval rate was only 50%. Banks are very strict now.

- How much credit history do I typically need for mortgage application ? Can RM 300 of credit history a month for petrol help to qualify RM 2000 monthly installment for mortgage? (assuming that I qualify for other criteria)
Amount spent is not that important. Importance is on the repayment trend and amount owed to the bank.

- Having credit history might help out for mortgage application but it also reduces your disposable income for monthly installment. Is there an optimum point?
No you managed your life yourself

- How long of a credit history do I typically need for mortgage application (3 months / 6 months / 1 year) ?
CTOS and CCRIS keeps records I think up to a few years. Can't remember

- Is credit card the easiest way to obtain credit history ?
Credit card and PTPTN or car loan