Yep you'll have to remove the splash guard. Remove the rear wheel, then warm up the splash guard a little to make it so it's not so stiff. I use a shop light myself. You'll see several plastic push-pin retainers at the very top of the guard and lower ends of it. Pull those pins and the splash guard can be moved out of the way so you can have access to the nuts in back for those without captured threaded inserts.
__________________ When you have a choice, buy American.
I would say you need to take off the splash guards, but you might not be able to get them back on unless you buy new push-in type fasteners. If you don't take them off, the nuts will probably fall out of place. Are you planning on putting them back on? When I took my flares off, I took out the splash guards and left them off. I personally think it looks better, I just need to spray some undercoating under there to protect the paint from rocks.