I didn't read the whole thread. Please forgive me if I've repeated something.
I own one as well. Used it for about a year before I installed the Victron BMV. It measures both ways and "net," i.e., if you were to have a load and a charge at the same time, it reports the net of the two, e.g., 10A load and a 20A charge would should as a 10A charge.
Do not connect backwards.
IMHO, you should program it for 140Ah and set it SoC to 50%. It should then count input current and hit 100% when you have input 70Ah. If you leave it connected during use, it will count current out and indicate 0% when you've used the 140Ah programmed.
My only real objection (what can you expect from a $50 BM?) is the unclear charge termination criteria, i.e., what resets it to 100%? If it's in the video, I didn't watch it, and I never figured it out from the instructions. I would just periodically set it to 100% when my batteries were fully charged and in float mode. Based on a comparison to batteries at or near a resting state, it was close enough for me.