That reminds me that you could do a test print at ShortrunPosters.com:
I got an 18 x 24 print for a lousy $11 (including special coating for $2 and $5 shipping). Results are excellent (yes!). The only wrinkle is that they don't use offset printing; they use a machine called a Xeikon 5000, which is not inkjet and not offset, so it is hard to know. They only warn you to have at least 150 dpi in submitted images, so it is probably more forgiving that offset.
I think you should upsize the 2nd image (showing 189.45 ppi) and pixel-peep the results. I'll bet it will look fine, and you won't have to worry about the results (I think). For one thing, upsizing some isn't nearly as destructive as downsizing. ANYTIME I do a downsize (for the web, emailing, etc), I ALWAYS have to sharpen afterward, whereas whenever I have tried upsizing (seldom), it looks pretty much like the original unless you really push your luck.
I have read that using an even percent - like 200% - is better than fine-tuning to a particular dpi with a funky upsize %. I don't know if this is true. I think I'd do the funky percent because it would freak me out to do even 200%.
I am full of half-baked and semi-accurate suggestions on subjects I have very limited knowledge, so feel free to ask anything...
Olympus E-M10; Panasonic GM5
m4/3 lenses: Oly 75-300; Oly 14-42 f3.5-5.6 II R; Oly 17 f1.8; Oly 40-150 f4.0-5.6 R; Oly WCON-P01 adapter; Rokinon f7.5 fisheye; Sigma 19 f2.8; Pan 20 f1.7; Pan 12-35 f2.8; Pan 12-32