2. If Brown was uniquely bad for his carpetbagging issues despite the 2014 red wave, than Maggie Hassan would've lost to Walt Havenstein in the gubernatorial race and Annie Kuster might've lost to Marilinda Garcia as well in NH-2. But as Zaybay mentioned, the Democratic Party in NH is one of the strongest in the US. Compare that to the Kansas GOP, which is in a historically weak position.
The Kansas GOP is in an historically weak position? Not especially.
Since the days of JFK, the Democrats have
gained this seat exactly once, in 1998. That was the year the GOP seemed to be literally tearing itself apart, with the incumbent Republican governor facing a primary challenge from the state party chair and the freshman congressman in the 3rd district seeming to support the losing side. Meanwhile the Democrats nominated Dennis Moore, a former Johnson County officeholder with an established track record. Yard signs for Moore showed up in the same yard as signs for the Republican governor, and Moore managed a narrow 3-point win. (Rumor has it that when Congressman Snowbarger's loss became known at the Republicans' election night victory party in Topeka, there were mutterings of "Good, he had it coming.")
Republicans may be unpopular this year, even somewhat here in Kansas, but the atmosphere is nothing like that. Yoder is perceived as a bit of an opportunist, and he doesn't have a lot of friends, but he hasn't made a lot of enemies either. It's always possible something could happen to make the atmosphere toxic for the GOP--Kris Kobach is on the ballot, after all--but I don't think it's happened yet. This may be a district that Hillary Clinton won by 1 point, but it's also a district that Mitt Romney won by 8.
Yoder could theoretically lose if all the stars aligned perfectly for the Democrats. Having someone easy to caricature, like Brent Welder, as their nominee is not an ingredient for the stars aligning perfectly.
(Then again, you can't rule out flukes--I never expected Jim Leach's loss in 2006 or Rick Boucher's in 2010.)