First of all, Bashir clearly stated that Odo was infected in a Stardate 49XXX, which would be in season 4, and Children of Time was season 5, so Odo clearly was infected.
Is it that hard to accept that the writers just hadn't thought of Sector 31 at that time and forgot about Children of Time when they came up with the timeline of the infection? It's just a TV show, and every scifi show not featuring Edward James Olmos has plot holes.
Second, yeah, there's no reason the skills to mimic a better face would have transferred.
Third, before Abrams Star Trek clearly used the one-timeline approach to time travel. Those people never existed. The only scifi before the Abrams Star Trek I've seen which uses the 'alternate universe' approach to scifi is an obscure webcomic called Fans.
Edit: And for that matter, why didn't Odo make any attempt to cure Las?