One of the basic principles in the kabbalah of the Ramchal is the need to rectify (m'takein) the female aspect of the presence of God (the shechina) which is currently in exile among the husks (klipot). Furthermore it is basic to all kabbalistic systems that there are a multiplicity of worlds, each of a more spiritually refined nature then the next, with our own world being the least connected to divinity.
In Tikkunim Chadashim (also known as Shivim Tikkunim) the Ramchal explains that the idea that zechus avos is no longer available means that while in the times of the avos the rectification of the shechina was achieved above (in the supernal realms/worlds) with the ramifications issuing downward evetually into our existence, that is no longer possible. Hence any new rectification (which according to the Ramchal would come through the 2 moshiachs and Moshe) must be instigated from below and raised to the higher realms. See further Jonathan Garb's article pg 191 and note 19.
To summarize: The Ramchal sees the termination of zechus avos as a special quality of the avos which enabled them to to achieve unique spiritual feats which can no longer be replicated in the same fashion.
Thus to address your questions I would extrapolate two things 1) the merit of the achievements of the avos may still very well exist and benefit their descendants and 2)when we pray and invoke the avos we are noting both their accomplishment and the fact that we seek to achieve similar ends (a tikkun of the shechina) in our own way.