correct all there is a nut welded to the other side of the box section of the chassis rail, and yes you can gain access from the inside of the car, however you will need to open up the air vents like i said. i dont think they're there specifically for this, just conveniently placed as i think for put them there to promote rust like all of there othe cars

the easiest way is to angle grind the old bolt off if it's caught in the snapped nut and then using some fine ish wire drop the new bolt (want to be longish) down the into the box section and then get the mount on and nut. can use the 2 nut trick to stop the bolt from turning but once the nut is nipped up it shouldnt turn anyway

first time you do it seems fiddly but i've done it a few time and it's a piece of pish

Rob