And that's the problem, we don't know the droprates for gold dragons (or Manyus for that matter), we don't even know whether there is actually any variety in those rates between the different maps.
And with FKG, I wouldn't blindly assume that more stamina=better. Just look at the intermediate Secret Garden or daily gold maps, which aren't as efficient as the beginner maps. Or 13-X not being as efficient as 12-3 or 12-5. The rewards actually don't seem to be very important when the developers design these maps. They seem more interested in creating a puzzle-aspect and making the maps somewhat fun to do. Difficulty is mostly decided by the level of the pests, and the potential rewards scale with that. Avarage or minimal rewards hardly seem to be taken into consideration at all, only potential maximum rewards.
Well, from collective experience (my own and others here), what he states it true. the gold level 100s pretty much never (if they ever do, I've never seen it) show up in the Beginner, and are very rare in Intermediate, where over time they're rare-ish in Advanced.
That said, RNG loves to screw with people sometimes and adorn them other times, so if you want to give Intermediate a go and see if you get a lucky streak, by all means go ahead.
My own experience:
Beginner: 80/20/0
Intermediate: 50/45/5
Advanced: 30/50/20
Also, you say that 3/4 chests on Advanced can be missed but the chances of missing that many in the same run is absurdly low (ALL of your units would have to go through RNG nodes to the center-left, through the RNG node into the nests.
Assuming you use default setup and all possible paths have the same odds, the chance of that happening is:
Each of the top 3: 1/2 x 1/2 = 1/4
Bottom 2: 1/3x1/2 = 1/6
1/4 x 1/4 x 1/4 x 1/6 x 1/6 = 1/2304
Just saying.