A couple of things to point out...
First off, the stat references for Alfie were simply supportive of the recognition he has already received - he is regarded as an excellent defensive forward - elite by most in the Northeast division. When you combine his two-way play in the Stanley cup playoffs (let's face it, he was considered for the Conn Smyth despite Ottawa losing the cup) and there is ample justification beyond stats to put him in the top 20 in the league in def rating. The stats just validate and perhaps amplify his movement up the defenisve charts to top 10. Yes, he plays on a great line, but he is the defensive aspect of that line and his +/- dwarfs that of his linemates (i.e. Heater and Spezza don't do nearly as well in terms of goals for against than without Alfredsson, further validating his rating). Putting him top 5 is perhaps debatable, but worst case you're talking about dropping him to a 77 or 78.
As to challenges, remember that teams need to provide objective OPINION from reputable sources to succeed. While Alfredsson is regarded as an excellent defensive forward and sources like Forecaster regard him as a two-way winger than never takes a shift off and has impeccable work ethic, guys like Vanek and Huselius don't have this type of profile and usually categorized as lazy at the defensive end and often taking shifts off. As such, govern your challenges accordingly. Even for a player like Sullivan, be sure to read up his forecaster description (or others) before challenging, because he is not referred to as a good defensive player in any way, other than to say he is easy to knock off the puck.
And if we do get into challenging other teams ratings and we have the volunteers to run with it, the format I would suggest is as follows - You can challenge an individual rating on another team HOWEVER, if you do so, the other team gets to automatically challenge an extra player back on the team who made the challenge. This will ensure fairness in that teams making challenges had better be sure they don't have any weak ratings, because the counter challenge coming back is only going to make their team weaker. In the case of Alfredsson, as you can tell from this discussion, I'm only too happy to wade into a debate and provide no less than a dozen expert quotes on his defensive ability and counter back with a few challenges of my own, especially for a few of the teams chiming in on this board !!
) For instance, Bertuzzi's DI of 99 is obviously a joke as I can provide 20 sources that should help bring this down to a 50 and (a few minor penalties a game) while Forsberg's speed is now about 60, not 72, which puts him on par speed-wise with guys like Patrick Marleau (not in Peter's best day). Vokoun's DU is also much too high given his chronic injury status over the last few years, however this was likely missed given the DU doesn't account for lack of availability, but rather just games played. He should likely be a 55 or 60, which will mean he'll be good to play 40 games this yaer, not 65.
As to takeaways and giveaways, it is generally considered amongst the worst tracked stats in the league, which is why no league I know of uses it to calculate anything. Offensive players who handle the puck a lot and play aggresively in their playmaking and in terms of going to the net will always have more giveaways, but this has nothing to do with being good or bad defensively - it's about calculated risks, which are not tracked obviously. Meanwhile a sniper who never back-checks, but also rarely handles the puck other than to shoot, will have nominal giveaways despite the fact he is considered one of the worst defenders on the team. It's useless unless some interpretation is included, but even then, every team's official scorer treats it differently, which even make comparables useless.