The non-rounded corners in the preview is normal.
It does seem odd that the outlines of your cutouts are white in the preview. Did you create the drill holes with the circle tool or the mounting hole tool? If you used the circle tool make
absolutely sure those are set to Board Outline and not Top/Bottom Silk. If you used the Hole tool make sure your NC Drill file is included in the Gerber zip file.
It very well could just be quirkiness with the JLCPCB preview function, I've had some other abnormalities in the past that didn't show up in the end result. Quite a few of my Gerbers won't show a preview on the website at all but they work fine.
If you're ever in doubt you can use this online Gerber preview tool, it'll show you exactly what you're going to get.
I have a couple offline versions as well but this one is made by the designers of the Gerber format so I assume it's accurate...