Can I use Quikrete mortar mix for my mortar bed? I have already installed my pre-slope with sand/topping mix and sand. I bought all the mortar mix already and want to use it instead of taking it back

To be honest with you there are very few of us who recommend this type of pan any more. I haven't done one in 30 years. Some of the plumbers and even tile setters can chime in but I don't recommend a novice do something that I gave up 30 years ago.