Standover is overrated
1) I think stand over height is your least concern. You spend hours riding and only a second to dismount. You could easily dismount to the side. Even in sticky situations, you can learn to dismount off to the side, instead of hopping forwards. And if the situation is SO sticky that you cannot dismount to the side, you wouldn't be too happy dismounting forwards either.
2) If you have short legs, or a long torso, or long arms or all of the above, any bike will probably never have enough stand over.
3) Gary Fishers bikes seem to always have a ton of seat post exposed (mine does too). That is why even though you have a bit of post exposed, you still have trouble with stand over. I might be generalizing, but it seems that Gary Fishers tend to have low stand over height relative to the ETT. Can someone confirm this? If this is a fact, the 19in Cobia is the way to go.
4) Looking at your pictures, I think a 19in definitely is your size. It even seems a bit small!
5) Also, looking at the bike specifications:
the 17.5in ETT = 60.2
the 19in ETT = 62.2
there is only a 2cm difference between the frames. If the 19in ends up feeling too stretched out, you can just get a 2cm shorter stem. I would prefer putting a 2cm shorter stem as opposed to a 2cm longer stem since a short one gives you less of an "over the bars" feeling while going downhill. Or as customfab pointed out, you might even need a longer stem!
6) Finally, it seems that when i got better at biking, my body got more and more stretched and flexible, so i had to go a frame size up. But, you said you have bought a bike before, so you may have already been stretched out.
In the end, if the 19in feels comfortable, just ignore the stand over and get the 19in. Stand over is overrated, I always dismount to the side, even though I easily clear my top tube