Hi Carl,

Thanks for the questions.

You said:

• Is it true that if someone thinks an action is grave matter and does it with full knowledge and consent, then a sin, that would normally be venial, becomes mortal?

Yes.

You said:

• What if the actual gravity of an action was debated prior to going through it?
• More specifically, what if the person had debated whether or not the circumstances would make it grave matter, all prior to the moment of the action?
  

Constructive debate within a person would increase the capability of the offense, if it is committed. The action itself is a sin. Temptation is not a sin but that bastard satan trying to get us to sin.

You said:
As the person sinned, (all in a period of five seconds), they did have a thought that it was a mortal sin and most of the debate occurred after the act took place.

From that point on, the person struggled to actually trace their thought process during their sin.

• What should this person do?

If they are unsure whether it is a mortal sin or not, treat it like a mortal sin. Our Lord knows all and will give you the required grace in Confession not to commit that sin again. If one is having a hard time making a firm purpose of amendment for any type of sin, ask for that in Confession by saying something like:

Father I would also like to ask for a better firm purpose of amendment not to commit [blah-blah-blah] habitual sins.

I hope this helps,

Mike