I didn't bother watching your video any further than when you opened your equipment window.
So your gear is basically garbage. Let's take a quick run through it.
You don't even have your gold explorer emblem, and that's not a matter of "oh I don't have the damage woe is me".
All four of your rings are crap, two of them could be replaced with a silver blossom ring and ifia's ring prior to getting gollux rings.
Trash pendant(I'm ignoring the second pendant slot because it expired anyway), could replace it with a solid gollux or even horntail pendant and you'd be better off.
Junk earrings, replace with dea sidus to get an improvement.
I can't tell what that badge is but it doesn't look like crystal ventus(no boss set bonus) and you're much too low a level for the sengoku hakase badge or even ghost ship exorcist(which you can't get anymore anyway).
You appear to have three pieces of the muspell set(overall, gloves, shoes) while using a 120 hat and what appears to be a necromancer pistol, so you're losing out on a set effect bonus there
Face accessory, I have no idea what that even is but I know it's not the one from zakum so I know it's crap. Same with the empty eye accessory slot, get the one from zakum.
I don't know what that belt is either, but a solid gollux belt would be an immense improvement
No clue what that cape is either, assuming low-level, barely-any-stat slot-filling trash.
Oh and you're 138 rather than the 140 you claimed in your original post. Get two more levels, get a reinforced gollux belt and pendant(should be cheap enough depending on your world), get a full pensalir set(hat, overall, weapon, glove, shoe, cape).
In short, your gear is, essentially, trash. Work on that and you'll have a better shot at clearing bosses.
Oh, and as a counterpoint to your claims that the bosses are impossible at your level, I was able to clear them on a cadena with minimal gearing(not gonna spend big money on what is ultimately going to be a link/legion mule), so it's absolutely not the bosses being impossible and needing their health reduced.
No burning event, not even this tera burn, has given exp and drop coupons and a teleport rock. I don't know what you got on scania but it had nothing to do with the tera burn.
Root abyss is fine. There's this crazy thing called "it's designed with the assumption that you'll party with others in mind". Being able to enter as a one-man party is a fairly recent change, about a year or two now.
Tower of oz has been kneecapped enough by way of its rewards being added to marvel machine, it's fine.
What the hell problem do you have with mushroom shrine? It's literally space-bar-holding-simulator-2017 until you get to the bit with nue and the DDR bird at the end.
Monad is just buggy, sort that and it'll be fine.
Alishan train is not glitched, rather the explodey bits begin as soon as you enter the map(whether you're loaded in enough to move yet or not), easily avoided by holding left as soon as you enter the portal.
You should be able to, yes. I actually haven't heard much in the way of confirmation of it, even now, because it seems like most people are using the 10-200 burn to scratch a link or legion mule off their list.
