My reason for chucking the barrel is that those cracks are long, and longitudinal. For all we know, they go all the way down both ways, and unless you know exactly how far the cracks extend, you can't know you're cutting enough of the flawed material away to prevent the cracks from spreading. Hence my suggestion you take it to a gunsmith or machine shop which performs magnetic particle or dye-penetrant testing to check exactly how far those cracks go, before deciding to cut down and re-use that one.

I could do the dye-pen. full length testing for you for $60, if that option interests you.

The oly system uses a special barrel extension incorporating the pistol-cal feedramps, onto an otherwise-generic AR-style barrel. I don't believe they'll interchange with 9mm barrels from Colt-style 9mm conversions, which don't use a barrel extension at all, and build the feedramps into the magazine block. Colt-style barrels have a basically-bare ended, chambered breech face, and an indexing rim machined into the barrel blank for fitting it into the upper. Any replacement will most likely have to come from Olympic, or be a custom job reworking another mfg's 9mm barrel, or a 9mm rifled blank into something which can accept the Oly pistol caliber barrel extension.

I believe Olympic does sell just replacement barrels, even for their pistol caliber uppers, but you'd have to call them directly to get pricing and order, as they're not regular "catalog" parts. I'm pretty sure that if you faxed or emailed them a copy of your SBR registration, they'd sell you a new, shorter-length replacement barrel assembly for somewhere between $200 and $300, based on their "standard barrel" 5.56 pricing.

Possible warranty issues you'd have to take up directly with their customer service. Oly has a lackluster web presence, so you'll get your most immediate response by calling them.

There is a lifetime warranty on Olympic arms parts and components, but it doesn't cover barrel obstructions if that's what turns out to have caused your crack. They additionally have a 60-day satisfaction guarantee, but only on stuff purchased directly from them, and you're past the 60-day window anyway.