Personally, I take any CSA response with a heavy dosage of salt. They're typically hired for their abilities to be personable to irate and confused strangers than their understanding of the whys and wherefores of a company's products, policies, and intentions. Often times their only training is to be handed the same material you have to read over. And chances are, you have more experience with the game than Joe Paycheck in customer service anyway, and neither you nor they have the word on what R&D fully intended -- which is likely why your CSA gave you the note that they were passing your concerns on. So, as much as you like to throw around "Stealth as Intended," I think I'll take that phrase with an equal helping of cardiac risk. Your repeating the label doesn't make it true. YMMV, obviously.
As far as allies granting concealment, there is no mention of this in the description of "Line of Sight" on page 273. As noted, it is an exclusion-based system, and while many things are listed as blocking line of sight, there is no exclusion given for allies hindering line of sight. I would interpret this to mean that allies do not block line of sight, and that they therefore provide no concealment for a stealth check because the enemy can clearly see you.
That said, you are correct that there are still plenty of other ways to obtain concealment and cover on 4e battlefields. And I wouldn't want to use static checks for it either, which still puts you at lots of die rolls. Consider trying to discourage ranged stealth with in-game villain tactics rather than house-ruling the system; seems to me this would be the most fun solution.
And personally, I never understood why ANY non-fighter would desire to be up on the front line of combat. First rule of survival: don't get hit. What's WRONG with having a Rogue that doesn't want to risk his neck so needlessly? You say it likes it's a bad thing. Seriously, I'm playing a 4e Rogue now, and have no qualms staying to the back doing ranged damage, even without the extra 2d6 from Sneak Attack every round. Let the meat shields soften them up, and any that get through, well THEN you can flank-n-stab while they try to take down your 'casters ;P
Or are rogues just another squishy to you, and that's all they should be?
A better question is: what are a Fighter's incentives to stand up front taking all that abuse while the rest of the party lobs in damage that pisses the bad guys off enough that they pound on you harder? Because, yeah they're useful to the party, but they're also the most prone to PC death, from what I've seen. I'm not sure that this is a question to be answered with a game system, it's probably more of a role-playing solution.
-Dan'L
-Dan'L