In the Curse of Strahd module, it doesn't give an exact year that these events take place. Everything can be kind of fluid because of the nature of Barovia, which I won't get into because that might be a bit spoiler-y. However, according to the module:
Strahd had all the female dusk elves put to death around four centuries ago as a punishment for Patrina's murder.
In terms of Barov's handling of the dusk elven royalty, again, no hard number of years, but in some information about Rahadin in an Appendix, it states:
Rahadin, the dusk elf chamberlain of Castle Ravenloft, has served Strahd's family faithfully for nearly five hundred years.
So it's safe to assume a range of four to five hundred years ago for those events.
As an aside, fantastic character development potential. I would be super excited to play with/DM for your PC.
Wood Elf whispers to Goliath "Go hide somewhere else"
Yes, you have the right of it.
It is on the DM how powerful Mask of the Wild is. You can make it useless by hand waving that all forests are heavily obscured, or not obscured at all. Or you might make all forests light obscurement, making it very useful.
It is all about how obscurement affects a character's ability to use stealth. Normally, once you have been seen, unless you are in or can get to a heavily obscured area you can not re-hide. Wild elves can hide even if someone is watching them with only light obscurement.*
HIDING
The DM decides when circumstances are appropriate for hiding. When you try to hide, make a Dexterity (Stealth) check. Until you are discovered or you stop hiding, that check's total is contested by the Wisdom (Perception) check of any creature that actively searches for signs of your presence.
From the Stealth section of the PHB
What is lightly or heavily obscured?
A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.
The DM is there to make a call on what kind of obscurement the environment offers. It is not necessarily given that any character will be able to hide behind trees. I always think of a pine forest. Tall skinny trees with very little branches below 30 ft or so. A moderately dense forest like this would probably only be lightly obscured, or even mot obscured. So while the rest of the party be seen after they attack, the wild elf can use the sparse ground cover to "disappear" back into the treeline.
The good news is that it doesn't have to be a decision made ahead of time. When you get asked about how heavily obscured the environment is, make a roll. Vary the probabilities based on the environment.
eg. This jungle is lightly obscured one time out of 20, otherwise it is heavily obscured. This changes every 100 yards. OR Springtime rains in this region normally roll in in the early afternoon, and last for 1d4 hours. Most of the time there is a heavy downpour, ,making the area lightly obscured (1-15); sometimes it really opens up making everything heavily obscured (16-19). On a roll of a 20 the rain has eased up to a light drizzle. This changed every 30 mins.
If you decide to make it random beforehand; and tell the players, then you have consistency and fairness, more realism, with less work. And you have it right there to refer to anytime you get asked. After all, even the densest of jungles have clearings, and the lightests of fogs may have thick patches.
*This has all kinds of rule caveats and DM decisions surrounding it. I really recommend reading all of the following PHB sections for more on obscurement and stealth:
Vision and Light
Stealth - especially the hiding sidebar
Also this podcast gives some nice DM tips on running stealth.
Best Answer
Yes - the MM and DMG give tips on adapting the stat blocks.
On page 342 of the MM, in the "Appendix B: Nonplayer Characters" chapter, there is a section about "Customizing NPCs". It states:
Furthermore, the DMG has a section on custom NPCs on pages 279-283. On page 282 specifically, there is a list of various races (including some monstrous ones) and their corresponding stat changes, so you get a quick overview instead of having to comb through the PHB.
Note that when using this table, if you change the ability scores of an NPC, you might have do adapt his CR, as stated on page 283.
In your example, the scout stat block specifically lists as its race "Medium humanoid (any race)", emphasis mine. Consequentially, I would give the scout the corresponding racial traits (such as darkvision), but not change the given ability scores.