The second definition puts you on the right path. This utterance is used as a signal that the (working) day is done, and those present can be satisfied in the notion that their work constitutes a complete shift.
The alternative to this indication would be someone actually saying:
OK chaps, that's enough work to satisfy the definition of a workday. What do you say we all think of it as such and go home?
Person A seems either to be confused or to be using poetic language, as the meaning is not clear in itself but seems to reference a number of meanings and idioms:
Merriam Webster gives the following definition of "pool":
3*pool* noun:
4 a readily available supply: as
[...]
c : a group of people available for some purpose
The meaning of "pooled resources" in definition 6 is related:
6 a group of journalists from usually several news organizations using
pooled resources (as television equipment) to produce shared coverage especially of events to which access is restricted
In 4, "pool" is used to describe the group, rather than the individuals (like water in a swimming pool). The idea of "pooled" in 6 is similar: resources have been collected together (again, think of water in a pool).
So it seems that Person A is wanting to refer to a collective group of people committing false marriage, but they may have mixed this up with the ideas of
A
opening (oneself) up to (something), meaning that you are making
yourself available for something to happen to you
and
B a blowing something (wide) open (or blowing the lid off
something), meaning to expose the truth of something (usually in
reference to something scandalous): "I want to blow the lid open on
false marriages" would mean they want to expose the people (or "pool"
of people) who are having false marriages.
and/or b opening a can of worms, meaning to create a situation
that will cause trouble or be unpleasant
I think ultimately they used "pool" to give a sense that the group of people doing this (or who would do this) is quite large, and/or that they are a collective - a group with the same ideas and principles rather than individuals, AND that marriages would be "opened up to" a situation in which they happen falsely (for financial gain, etc) and/or that gay marriage equality would be like "opening a can of worms", which would cause problems.
Best Answer
After further research and enquiry, I've come to the conclusion that the closest interpretation of establish in the definition of entrench is bring into being on a firm or stable basis or to cause (someone or something) to be widely known and accepted.