can we gain access to beaches of our choice by walking through any hotel on that beach? if "yes", can you suggest any particular hotels on the prettiest beaches in cabo, mazatlan,PV, ixtapa,acapulco, and manzanillo...............

Maybe not, especially if they are All Inclusive hotels. All beaches are public and in hotel areas, they are meant to provide an access walkway to get to the beach. Not all do, money talks !
I can only speak for PV, but Playas Gemelas is one of the best beaches here. two major buildings on it the Intercontinental Hotel and Condos Playa Gemelas, there's a beach access pathway to the north of these buildings.

No, you cannot. This is from my experience in Puerto Vallarta and Nuevo Vallarta. I cannot speak for all areas. Many of the hotel and condominium people (especially all inclusive) try to tell you their beaches are private (although all beaches in Mexico are Federal property and therefore public). They will prevent you from access to the beaches via their hotels, but there are places where you can access the beach and then walk up the beach to wherever you chose. They cannot stop you from using the beach, only from passing through their grounds. Most of this type places have guards positioned at both the beach side and street side of their properties. They are adament about it and not very nice.

thank you! therefore, it may be beneficial to pay a "day-fee" and use hotel/beach facilities. does anyone know of hotels in cabo,mazatlan,acapulco,manzanillo, that offer day-passes?

Medano Beach in Cabo & Playa Los Muerto in PV are both beautiful public beaches lined with beach bars, facilities, activities, vendors, etc. where you can spend the day (if you don't want to go to a resort or pay a fee).