1) you do not need to weld the wastegate flap shut, the wastegate only opens due to pressure on the diaphram, with no pressure it shouldnt open, a mate of mine did the external gate thing with an internal gate, he had no actuator and just used a spring to keep it closed.
2) the easiest (and dodgy old school) way to adjust your wastegate is to bend the rod, the other way would be to cut it in half and put a thread on each half both in opposite directions (one left hand thread one right hand thread) then get a hollow rod of the right size and put opposite threads in the middle of that, the screw the rods into the middle of the hollow rod when you spin it one way the two inner rods will be pulled together creating more pressure on the diaphram and needing more pressure to open it, turn it the other way and it losen's it needing less pressure to open it.
3) the t3 adapter you mentioned is no doubt the one in the for sale section, 2 problems that spring to mind 1, will this be so big that the turbo now hits the strut tower 2, how will it effect the dump pump and lines since it is pushing the turbo 2-3" out from the engine, but if there is no problems and everything fits then i would go with that option.
4) once again you mentioned boost fluctuation with temps and other factors, the only way to make it accurate is external gate, the internal gate just does not have the flow and accuracy that an external gate has, there is also a big reason why an internal gate costs $100 and an a good external costs $500, you also have to look at the future, no use spending a few hundred on band aid fix's when down the track when you start serching for more power and better response your going to wish you had have gone this way to start with.
These are just things i have learnt over the years, cheap fix's produce cheap results that later you need to spend more money on keeping it going, do it right the first time and your set.