OS4.5 was available on the BlackBerry site yesterday, but was not available on the Verizon site. I haven't looked today. I suggest you continue to monitor both sites. With 4.5 for the 8830 appearing on BlackBerry's site, it should be imminent on Verizon's.
No, as long as it matches the device, you can get it from any source, including RIM. You just have to delete the vendor.xml file if you get it from a source other than Verizon. I prefer to wait on Verizon, but that's just my preference, not a requirement.