Is aluminum pressure cooker reactive? Will it give “off” flavors

There is no simple conversion, as you are looking for outcomes.

Typically what can be done in a pressure cooker for stock or braise type applications in 15-30 minutes might take 3 hours on stove top. An hour and a half in a pressure cokoer might be analogous to a full day (call it eight to ten hours) of more traditional cooking. Doing so in an oven (300 F, 180 C) is even better than a stove top, as there is less risk of scorching.

A good quality recipe will give you a test to know when an item is done. For example, meat falling off the bones in a braise. Stock is one of the few things where its hard to give a specific test, but you should at least have good flavor development, and thinner bones will become flexible.

90 hours is simply unreasonable for almost any type of cooking whatsoever. Perhaps they meant 9 hours, which is in the ballpark.

I suggest finding a better source recipe; if this recipe is far off on cooking time, how many other problems does it have?

The biggest difference I know between stovetop and electric pressure cookers is that the electric ones take much longer to release pressure after cooking. The ones I've seen/compared take roughly twice as long (10-15 minutes for stovetop, 20-25 for electric). Biggest reason I can think of is that you can move the normal pressure cooker away from the hot stove, but you can't move the electric one away from its heating element.

As a result, I would adjust the cooking times down by 10 minutes or so to compensate.

In regards to the high/low heat, the main advantage of electric pressure cookers is that they are more energy efficient, and are designed to distribute heat well. You shouldn't need to worry about heat variance--get it up to the pressure you need and start the timer.