what is the brand of instant hpw? and vit and minerals? It doesn't seem like a brand can matter, but it can. There are more than one hpw diets and sources for it, one is crap ( exotic nutrition) the others are fine.
I don't feed hpw but I don't know anyone that adds a secondary diet to hpw, nor do I know anyone that uses Mazuri. I wish I knew more about it and could help more.
The way you described their feeding plan it sounds like PP diet that has been modified by the previous owner. If the Hpw is Glider R Gravy that's a good staple on it's own and doesn't need modified with the Mazuri. Glider R Gravy is the Critter Love hpw repackaged for Pocket Pets. Also if that is what the hpw is you don't need the vitamins and you can feed more fruit than just apples.
Don't feel inadequate, diets are super confusing, and even more so when someone advises you to use a modified version of an established diet. There are a few folks here that feed Hpw's I'm sure they will have better advice on this specific diet.