Is it possible, after selecting a card (card action pack 1), to modify its variables (using idenpendent card variables)? Something to simulate Yu-Gi-Oh equip cards?Yes, that is what one of the Card Actions in Independent Card Variables is set to do. You will want to use these to achieve it:
Selected Var(X) set Y Selected Var(X) mod [+/-]Y
Taking this opportunity to also ask if it is possible to modify a card variable depending on the number of specific cards and/or a type in a zone?
Yes, you would need to make use of Conditional Card Actions to check how many of a specific Cards or type there are in a Zone and then change those variables on a Selection of Cards.
Let us know in this thread or on the Discord if you need any help setting these up. We can troubleshoot it with you :)