Hi Rob
Sorry to hear you are having problems.
Are your Drills ground finish and do they run true in the drill chuck?
I have had drills that were totally useless because of being bent.
Drill the holes @ one sitting ie just go for the 4.2mm.
Please don't start drilling your frame until you have perfected your marking out/drilling & tapping technique.
Also you have made a nice job of tacking your plates onto the tube, if i was going to finish weld i would only weld the top & bottom of the plates, if you weld down both sides you will get weld shrinkage that will cause them to bend.
Regards
Mike