As a retired qualified boilermaker with 35 years hand on experience, during my tenure at that also held a coded welders ticket, I will give my humble view on this.
Plate C in my view was the best piece looking at post one without even looking at the other pics posted. Fusion was excellent with parent material, the cap really very low / flat that will lessen HAZ in a big way. On the flip side of the plate one can see that the penetration was good.
For the non professional welder; big fat caps / beads are not stronger, in fact the opposite is true. When I still welded as a coded (X-ray), the max allowable cap was only 1.6mm and that's on big bore pipes. Bigger caps were failed due to the unwanted stress resulted from that.
I agree Bonzo, it is better to weld rather hot than cold,. The common sin of most amateur welders are the fact that there current is to low. A simple trick to set one's welder to a specific electrode (rod) thickness, is to multiply the diameter with 40 and that will put you in the range of a good weld - vertical up welding needs some downward adjustment if needed. Eg; 2.5mm (12#) electrode x 40 = 100amps.
Furthermore, Im always skeptic of nicely grind-ed welds as your pics shown for the simple reason you hide the tale the cap has to tell...
A nice bead such as "C" looks more professional than a grind one and I would rather see such a weld bare.
At the end, a destructive test - like the bending test you did - will be the decisive factor for quality of weld.
Lastly, kudus to you for the effort to post this excellent post and the trouble you took to do it. I believe it is an eye opener for a lot of people not well versed in welding strength