My group is about half way through this adventure now. A few comments/suggestions based on how the adventure and campaign have played out so far:

By now, 2 1/2 modules in, the players have come up with a lot of questions. I've used Leena to answer a lot of questions, having her better explain the relationship between Modra, Sarshan, Myrissa, Tusk, and Iranda. Essentially, she's not really providing much new information, just clarifying/confirming what they already knew. For instance, by letting them realize that nameless Dark Creeper in The Vents with the key was "Iranda", and that was Modra's partner in crime. I've also had her foreshadow Thannu, and speak of Sarshan's 'tremendous power', so that Thannu seems more like a legitimate "end boss" in the tower, and less of a bait-n-switch consolation prize. My hope is that they'll actually work toward killing Thannu while avoiding Sarshan, figuring they'll have to content themselves with disrupting his operations for now, so that his escape is more their choice than a ride down the railroad.

Did anyone else find it weird in the Dark Foundry encounter that you had a 3x3 creature in a room with several 1 square wide bridges? How did you handle it? I just let it cross, though I did make Acrobatics checks every time (it made them every time). If I were to run this again and not use printed maps (which otherwise are really, really nice to have), I'd probably make the center bridge wider and reduce the Acrobatics check on that one. That way, there is a more plausible way for the boar to cross as well as a less treacherous pass for the workers. Leave the other more dangerous bridges to provide a risk for trying to get flanking, etc.

In the Training Grounds encounter, I recommend encircling the battle area with goblin, orc, orge, troll, and other monstrous minis to give it a backyard arena feel. I described various factions cheering for different teams, dark creepers running around taking bets (a good way to provide a monetary treasure parcel, the bookie gives them their cut). I even had two audience members get into it and start fighting (each other).

The complexity 4 skill challenge in Umbraforge got tedious by the end. I would either drop the number of successes or break it into a series of complexity 1 skill challenges in the various parts of the city: training grounds, mercenary camps, slave bazaar, market, etc.

Good catch. Throughout the adventure path, there are often little inconsistencies or problems like that which went unnoticed before publishing. I hadn't picked up on that, and I was about to run the encounter tonight. I plan to use a 2x2 mini for the boar though, so it won't be an issue for me. (I'm playing with only 4 players instead of 5, so I have to 'downgrade' encounters slightly anyway.)

Regarding a 3x3 creature and 2x2 spaces though, there are rules in the PHB for "squeezing", pg 292. The boar moving across the bridge would incur penalties, but be very doable, and using the bridges to slow the creature could very much become a part of the party strategy.

Regarding the skill challenge: this is my/my group's first campaign in 4th edition, and this module was the first exposure to complicated skill challenges. The players I have are keen on roleplaying, but still found these challenges difficult at times. I find I have to be very flexible, as a lot of their solutions are 'outside the box', and then simply find ways to fit them in, figuring out which skill is most appropriate and asking for a roll. Even then, I cut the Umbraforge on short - as you said, it was tedious by the end. Fortunately, ours played out that the training ground was the last place they visited, so I played the crowd and combat, and had Leena stay behind after they bested the ogres.

I ended up downplaying Leena and even Reniss, to a point. As written, they feel too deus ex machina for me. I actually introduced Reniss partway through that skill challenge, let my players choose her as an ally, then split up and go find out more info. Especially for the Umbraforge encounter, if you break it up into sections, I would end each section by giving the PCs instructions to talk to someone in another area of town, ultimately leading to a meeting with Leena. With two combat encounters in the mix, I don't think it would feel too messenger-boy-ish.