2 - You can't connect walls at a non-existing midpoint. To make this work proceed as follows:
- use Split wall to divide wall A in two parts - Aa and Ab
- use exact X,Y coordinates to position the split at the point where wall B should connect
- make sure the wall directions are correct (suggestion: wall Aa and Ab go from left to right, wall B goes from down to up
- select wall B, then wall Ab and use Join Walls to connect them

I tried your solution.
I was able to connect wall B and Ab and both got connected. However, I have two queries now:
1.Aa wall gets separated . is it not possible to make a T where all 3 walls are joined?
2.Inspite of splitting the wall at 3', it got splatted at 2'6" & 6'6"

Please see the figure below
Fig A - Original position where B was separate
Fig B - Split was done at 3'
Fig C - B and Ab selected
Fig D - Join successful
Fig E - Wall A got separated whereas i wanted a join of all 3 walls and another issue is that, 3' turned into 2'6"