I am familiar with MEC stuff, this bike is a nice bike, but the crankset choice was dumb, the low as it comes stock is probably about 27 gear inches. The use of a standard "bread and butter" 50/39/30 crankset is probably a mix of marketing reasons and/or price.

My bike, a Tricross, came with pretty much the same gearing, 50/39/30 and a 11-32. My rear derailleur is slightly diff than the Tiagra one on the National, but the easiest way to lower the gearing is to change out the small chainring from a 30 to a 26. The big advantage of the National using a 10 speed 12-30 cassette is the really nice smaller jumps between each gear. Im pretty certain it is 12-13-14-15-17-19-21-24-27-30. I have put a 12-27 9 speed on my bike for all non "loaded" riding, and the spacing between shifts is really nice, the 10 speed has the same except the 30 is added at the end.
I changed the 26 tooth chainring with no adjustments to the fd, easy peasy. People at the store may tell you its not doable but it was no issue at all. You will however want to check the specs on the tiagra rd , compare it to the Deore LX on my bike , just to see how it compares. Really, the only issue is that if in the 26 "granny", you may not be able to ride in the rear 12,13 or 14 teeth cogs of the cassette, but you really shouldnt be doing this anyway, and its not necessary by that point, you'll have gone back to the 39 tooth chainring by then.

re: being told by staff what is doable and what is not, remember that MEC employees are not bike store employees, and so can be working in tents one week, then bikes another, so take what is said with a grain of salt (you can get this in any bike store, wrong answers I mean, but perhaps more so at MEC)

the only downside to changing the 30 to a 26 is that when you downshift from the 39, you may find you have to go up a gear or two in the back right after. For me, this is not an issue, and I appreciate the lower gearing, plus the fact that it cost the price of a 26 tooth chainring (under $25) as I did the changeover myself.