Why do you think a draw is the least likely result? I'd say it's likely enough. They're both fairly evenly matched. Croatia are ranked 18th and Ukraine are ranked 24th. Croatia beat Ukraine 1-0 last time, although the Croatians had home advantage. Ukraine will have home advantage this time, so that may offset the slight edge Croatia have over them in ability.
I think we've a pretty good chance of Scotland failing to beat Slovenia in Ljubljana. I'd say there's a better chance of that happening than Greece failing to beat Cyprus in Nicosia, but the latter is very possible too. I think we can be confident of B&H failing to beat Belgium though. A B&H win in that fixture would be a major surprise, even if they're at home. Belgium are unbeaten all campaign and beat B&H 4-0 first time round in Brussels.
We don't need B&H to fail to win both their final two games. We just need B&H to fail to beat Belgium (and Greece to fail to beat Cyprus, of course). B&H are on 14 points in their group right now. If B&H draw against Belgium tomorrow, it'll leave them on 15 points with one game to play. If they lose, they'll remain on 14 obviously. Even if they then beat Estonia in their final game, they'll only have a maximum of 18 points, which will translate to 12 points in the runners-up table; one less than our runners-up table total of 13 points, if we beat Wales.