(edit: I decided to simplify the question and only pose it for bounded posets first)

The Union-closed sets conjecture is equivalent for lattices P to:

>>There exists a join-irreducible element $a$ with $|[a,M]| \leq |P|/2$, when $M$ is the maximum of $P$.

Call a (finite) bounded poset $P$ lattice-like in case an element $x \in P$ is join-irreducible (recall that an element a of a poset is join-irreducible if there is no subset $X \subseteq P$ and $a=\lor X$) iff $x$ is covers a unique element.

Every lattice is lattice-like but not every bounded poset is lattice-like.

>Question 1: Is the above conjecture also true for lattice-like posets?

This is true for all such posets with at most 8 points. I would think there is a counterexample but I have not found one yet.

>Question 2:Are there attempts in the literature already to generalise the Union-closed sets conjecture from lattices to a larger class of posets?