Thanks OP for posting this and giving me the confidence to tackle it myself.
Beejayzus what a PITA (if you can't get to the rear of them) those square plugs are that clip the plastic inner guard in! Before I started, due to your advice, I tried to get some plugs from Repco and Supercheap but they had nothing to suit. I continued anyway and left the last few in near the sill panel and bent the plastic panel away to get inside the guard, this caused other issues...
Like you said, that last nut up the top closest to the windscreen sure is hard to get ones hand in to. I dropped a nut and it hit the plastic inner guard and rolled down between it and the sill. I put a socket on the next nut to help me grip them, then I dropped both and they joined the previous nut. I was then forced to mash another plug to be able to poke my finger in (whilst losing a layer or two of skin) to retrieve the two nuts and the socket.
$90 (delivered) snorkel from eBay, great quality, self install = winning.
Sweating so much in the hot sun that I had to take my crocs off as they were filling with perspiration, bark off a few fingers, several trips into town for bits and pieces, a new holesaw (and another arbor as the one I had would not fit!), a few mashed plastic plugs, most of the day in total, quite a few swear words = priceless.