Minha experiência

I recently ran Rise of Tiamat for a group of (mostly) 4 players. Like you, I added a fair number of extra events/missions/loot. I also changed a lot about how available the dragon masks were, I think positively.

In Rise of Tiamat, the adventure as written actually never gives the players the opportunity to take one of the Masks from the Cult before the final confrontation. The White Mask Varram is seeking is actually already at the Well of Dragons, the Blue Mask being studied at Xonthal's Tower is a fake and the real one is at the Well, and while the Green Wyrmspeaker is vulnerable to the PCs in the Misty Forest, he too doesn't have the Green Mask; it's at the Well.

(Hoard of the Dragon Queen does in fact give the players the opportunity to find the Black Mask - but only if they find it before they kill Rezmir, in which case, you guessed it, it teleports to the thrice-damned Well)

Reading through the adventure, I thought this was a huge narrative problem! These Masks are the keystones of the Cult's ritual to summon Tiamat, and many of the party's missions are focused around chasing down rumors of one to try and keep it out of the Cult's hands, but they always fail no matter what they do? I anticipated this would be very narratively unsatisfying for my players.

In the end, I rewrote things so they had the chance to take up to 3 of the 5 Masks from the Cult. They ended up getting all three, but the Cult stole one back. I also heavily modified the final ritual battle so that keeping the Masks from the Cult made the task of stopping Tiamat's ascent easier.

The Green Mask

One of the rewrites I did was the Neronvain encounter. It's fairly simple to just give the Mask to Neronvain under some circumstances, as you suggest, and let the players loot it if they defeat him. If they get the Mask, that's a really concrete blow against the Cult's plans, so the players feel like they're making significant progress.

(As for the encounter itself, the Mask didn't affect it at all. They ended up magically abducting and killing Neronvain before Chuth could even get to them)

While you might be worried about the power of the Masks, in practice they're not that powerful for being Legendary items. Legendary Resistance is a great ability, and damage resistance is good too, especially as they are likely to be facing dragons of that type, but the rest of the abilities are fairly situational.

One other pro/con (depending on your perspective) is that if you let them have the mask, they will expect it to matter, and may be disappointed if it doesn't, in practice. In the adventure as written, the only effect of the Cult not having a Mask is on Tiamat's strength if she is summoned (and even then, only the lack of 1 mask is relevant, lacking 2 doesn't matter more). If they actually stop the ritual, as written they will not notice any difference. So you may have to do some work to make them feel that they did achieve something by getting the Mask (I had the coloured flows of magical energy to the ritual be very visibly weaker/slower in the colours the Cult didn't have Masks for, and that genuinely did make my heavily modified ritual slower). Depending on your perspective, you may see that as a downside or an upside!

Em síntese

I think adding the Mask gives a concrete sense of accomplishment & stakes to this questline. I saw very few downsides from doing so when I ran it, and I would recommend this change to you.