In regards to the roaming pokemon, when is the shiny determined? I saw that it's the first meeting (so you can't find it and then look for it again and hope that one or future ones are shiny), but then someone said something about being determined at the beginning and then I see shiny hunts where people sr before they are awoken in the tower. I can't tell if you have to save before you wake them or if this is just their way of making it easier by going to the nearby routes right after they've awoken. I've awoken them in my gold but have none saved as being encountered yet so I should be able to still shiny hunt them right?
Also could someone tell me what place in Johto has the most variety of pokémon in a single encounter area? I'm shiny hunting and want a lot of variety in the patch of grass, cave, etc I'm hunting in. For example in gen 5 Celestial Tower only has two species there so that would obviously be one of the worst areas for what I'm trying to do (I know gen 5 is different I was just giving an example)
For the first question, I tried looking for an answer but found the conflicting information you mentioned (leaning more towards "save before you awaken them", though). So, I don't know if we can tell you whether you'll still be able to shiny hunt them on your Gold.
For the second question, Route 38 could be an option, with up to 7 grass encounters depending on the time of day and version: https://www.serebii.net/pokearth/johto/2nd/route38.shtml
I don't know for sure if there's another Johto area with more grass/cave encounters since I didn't check every area, but you could check Pokearth for other areas you're interested in looking in. I also just looked at standard walking in grass/cave encounters, so there may be better places to hunt if you're considering Pokemon found through Headbutt, fishing, etc. as well.