As for the first one.. I asked somewhere else and was given this diagram.
Are you enlarging the pattern to the grey line on the left and right? If so, that will make the neckline wider and push the shoulder straps further apart, and enlarge the armholes. If that's what you want and need, great, but if not you'll constantly be shoving those straps back onto your shoulders and wondering why everybody can see your bra under your armpit!
In addition, if you add those darts to the side seam without adding length to the front pattern piece, you'll find that your front side seam is suddenly too short and won't match the back side seam. The new dart at the bottom of the front pattern piece will also make the waist smaller, so if you haven't compensated for that (by adding width somewhere else) the front will be too tight around your middle. Releasing the original gathers may give you enough space, but make sure to measure the finished seam line with the dart folded before you cut your fabric. Ask me how I know this is important
You need to do a Full Bust Adjustment (FBA) to make room for your boobs without making everything else hang like a sack. I'm busty too and I know what it's like to suddenly lose my waist because everything is cut for my widest part. An FBA will add room for your bust and tummy without enlarging the shoulders, armpits, neckline, or side seam length. Google FBA and learn how to do it properly -- you'll never look back. If you are not confident in finding the right option for you, head on over to Pattern Review and ask the folks there, as they are very helpful indeed. Here is a great thread of FBA resources for you to start with:http://sewing.patternreview.com/SewingDiscussions/topic/9531
And yes, you should wear a properly fitting and supportive bra both while you're adjusting your pattern and while you wear your clothing. Don't wear a good bra to measure and then a poor bra to wear, or vice versa, because you'll be wasting your time doing proper adjustments and you won't be showing off your figure in its best light. Good luck!