I am feeling archetypes more acutely and adding spontaneous archetypes(S.A.) more. My problem arose when I felt I needed one after labeling the freehand.

What is the proper way to handle:
Add an aspect ([A], [B], etc.) label (what if I completely mucked it up and the S.A. is the target?) Or leave for element exploration after S.T.?

My suggestion to you is to not add them in and keep trucking on through the session. When you keep doing that, adding in an archetype, two things are happening: one, you are saying to your unconscious that it is okay to come back and change anything at anytime, and that is a big no-no. When you stop doing this and finish the session like you used to, your unconscious will learn to give you everything at the correct, appropriate time.

Two, It sounds as if you are perhaps doing your S3[site] too fast. I would suggest next time for you to slow it down a bit, say 30% slower and see what happens. Really 'search' the page for all of the applicable archetypes. Then follow example number one.

In my experience when I, post-session, believe that I missed an archetype: The information usually gets thrown in with one or two other aspects that I did label.

I would say that what you're experiencing is just growing pains. Keep on trucking Ross and post your session work!

This was an uncharacteristically fast Freehand Sketch(F.S.) and probably the source of this rare error. In this particular case I was able to act as if this was an Analytical Sketch, without thought, and played the next note.
After reviewing my notes, your post, and weighing the importance of the F.S. I am wondering if scrapping the session and starting over wouldn't be the prudent course.

Is the F.S. important enough to cause a restart if I feel it lacks after labeling?

The freehand sketch is absolutely critical to a successful RV session, in that one must correctly identify all key site aspects in order to further explore them. Definitely scrap a bad S3 freehand and redo it, or walk away for a while and restart the whole session.