hi, pi -
i've only done three of these so far, but it does sound like you did everything well -
the butt's i've got have been from wal-mart as well - probably not the best, but they've worked well; the injection didn't seem to hurt anything although i would prefer them without. the marbling is generally fine as it all renders out and is what makes the butt juicy and moist.
i generally smoke at about the same temperatures (short temp spikes are no big deal as it is a big piece of meat) until the meat is 190 degress, then i pull it off the fire, wrap it in foil and towels for about a half-hour and then pull and enjoy. i do intend to try foiling at 160-165, in order to see if that helps keep the surface from getting TOO dark.
my suggestion might be that going as far as 200 before pulling it off the fire nmight ahve been waiting just a bit long - give 190 a try since the temp still continues to rise after it is pulled off. as one fellow explained to me, "there's cooking meat until it is done, and there is cooking meat until it is barbecue. those are two different things."
yep, the carolina sauce i use is based on cider vinegar, brown sugar and red pepper flakes. i got the recipe from RIVET and it is good:
the red pepper flakes might sound scary, but they really do enhance the flavor without being too voer powering. if you choose to try this recipe, you can try reducing them by half, if you want, but i don't think it's necessary.
one final thing - along with the excellent advice provided here by forum members, i would recommend that you download this safe word-zip file called the BBQFAQ and save it to your desktop for reference -http://www.eaglequest.com/~bbq/faq2-w97.zip
there is a LOT of stuff here and will answer nearly all posaible issues ~ i can honeslty say that this thing has helped me turn out some great stuff.
above all, don't give up! give it another try, change one or two small things with your goal in mind, and keep track of results. it might sound a little crazy, but take pictures of the process, too. they can help you keep track of what you're doing - and besides, we love q-view! i've got two strikes against me - i've lived in the north all my life plus i've never done real BBQ of any kind until last year. if i can do it, anyone can!