The A's move for Shark made far more sense, regardless of what they gave up, mainly for 3 reasons:
1. They were the best team in the AL at the time (the strong get stronger)
2. They made the trade earlier in the season and have Shark for more starts this season
3. They also have Shark for next season at a very reasonable price - accordingly they can let him go to FA and take the compensation pic or trade him mid-summer to a contender for prospects
As far as a starter, I would honestly look at Tommy Milone from Oakland. I know they want him as insurance but I'm sure they'd move him for the right price (which would be much less then your proposal for Lester). In addition, Milone would be under control for several more years at would looks to be a bargain price.
I don't think their biggest need is pitching. Looking at this roster as is today, if they could upgrade any of 2B, 3B or CF I think they have to do it, especially if the player is someone who will be around next season. Looking at next season, it's very possible that Melky and Colby are gone and we still don't have a decent option at 2B. Any trade they make now, if it involves our top prospects, has to be as much about tomorrow as it does today. I agree the window is very small (especially considering their roster as it stands for next season). I want them to win right now too but, fact is, they may have to stand pat and hope this team plays the way it did in May going into August and September. Honestly, getting all of Lawrie, EE and Lind back (or any of them) will represent huge additions at this point in and of themselves.