Those are the right questions to ask, definitely.
1. If he stays healthy and produces as expected, then secures a fair contract from someone else, we should get a 3rd round comp pick back. 4th would be unlucky. So yeah, that also drastically mitigates the loss of the 2nd.
2. Avril hasn't showed signs of slowing yet, and 31 isn't as old for a DE as many other positions. He set a career high for sacks last year, and looked the part. Bennett seemed off last year, but he was battling injury and we ran him ragged. Funnily enough, this trade my help on that front too. One of the things the Hawks love about Bennett is that even though he's a wrecking ball of a DE, he could slide inside to DT on passing downs and let them pretend they have a decent interior rush. Unfortunately, this is a big ask physically for a guy who weighs 285 (most DTs are over 300). I imagine if Richardson can pick up the slack inside, Bennett will be a lot fresher and healthier this year. Again, the synergy of this move is just off the charts.
As for resigning Shelrich, I dunno where the money would come from. Cap manipulation is usually beyond my ken, but Pete and John are pretty elite at it. They've managed to keep the band together this long, losing only 2nd tier stars like Irvin, Maxwell, Tate and Okung. Personally, I imagine that Shelrich replacing one of the DEs is the most logical choice, given that they can still keep the other one and Frank Clark. Keeping everyone at once would be tough, as seen by the need to restructure Baldwin's contract just to fit Shelly for this year. I dunno though, if this fearsome foursome D-line carries us to a Superbowl, we may be obligated to let someone like Graham go instead, and try to keep the defense at historic levels for another few years. It'll be fascinating to watch.
3. Great point on McDowell. Our D-line looks terrifying this year, and that's BEFORE we get a passrushing DT who fell to the 2nd due to work ethic concerns, not physical ones. That dude is terrifying. His technique was horrid in college, but the raw physical ability is way higher than anyone on our current line, including Frank Clark. There are some highlights where his short area burst to the quarterback is horrifying, like watching a panther pounce or on hapless wildebeest. If he ever learns a decent club or swim move, they'll be saying "break up the Seahawks' and meaning it, too. Quarterbacks would become an endangered species with Clark, Bennett, Avril, Shelrich and Peak McDowell all gunning for them at the same time. Maybe we'd pioneer a 5-man defensive line in base downs against the Patriots and put a forceful end to Tom Brady's career...
Or, more likely, a successful McDowell return would make it easy to let one of the other linemen go, and stay out of cap hell. Either way, it would be huge. Let's hope he heals up well.