Anonymousbasic guidelines (if those are avaiiable)
Oh. I see what you mean better now. Unfortunately, I don't know of any guidelines like that. Maybe other members of the forum can help you on that.
As for your specific examples,
We had many discussions ... (countable plural) seems best to my ear.
We had a lot of discussion on ... or There was a lot of discussion on ... (uncountable) are also possible.
In the second example, I sense that only a countable version of sin is possible because specific examples were given and a possessive adjective is used.
Multiple sins were mentioned, so you need sins (countable):
... because of your past sins.
If only one sin had been mentioned, you would have needed sin (countable):
... because of your past sin.
If you wanted a generalized idea of sin (uncountable), you would need a different phrasing:
... because of sin.
CJ