How to interact with a module from a different module

Do you mean data validation on QLI save?

If the QLI record is related to any Priceline (affected by priceline), then any additional editing of quantity or discount  shouldn't be saved to QLI (cannot be valid) - at least until  QLI is unlinked from priceline or QLI Product is changed. Yes?

No. User should not be prevented to edit QLI manually, as he wishes. And it's uni directional. Just module custom_pricing has to be stopped from executing it's code if pricelines were already used (not all products use pricelines, that is why simply trying to detect if pricelines exists is not enough). Custom_pricing triggers it's magic on the click of a custom button inserted in the QLI layout.

Okay, could you express expected system behavior as a use case or as a user story?

What do you mean?

When I select a product using the thing circled with blue, it automatically applies the "price lines" it has a price line associated to it. If this happens (not all products have price lines associated), that the button circled with red should detect this and NOT apply it's own calculations.

If I remember correctly if the user populates QLI.LineItem with a value from  ProductCatalog in QLI editing interface, SugarCRM relates the QLI record with ProductCatalog record specified by the user and copies ProductCatalog.UnitPrice to QLI.UnitPrice

Therefore, expected behavior might be the following: in case QLI.UnitPrice is not equal to 0, and the QLI.LineItem is populated, do disable the "Calculate" button in the QLI editing view

AFAIK, developers can implement such the use case requirement in JS Sidecar 

Why do you need to have both custom modules, can't you build the pricing rules into one single module and an appropriate logic hook instead?

Would be easier to bug fix and maintain in future as well.

I can not, because the business intends to sell it as separate optional stuff. Customer can either buy only module "A", or module "B", or both "A+B", or neither

You can still sell based on the enabled functionality/options all in same A+B single module

Erm.. even if that would be the case read above. It is not sufficient to detect if module "A" exists. I need to detect if module "A" was actually applied to the product that I am about to add to the Quote.

Maybe I missed something, but if you only have module 'A' then doesn't that make it simpler than checking if both A and/or B exist and all the other additional checks to see if one or the other has been applied?