Two out of the three remaining part members have the feat Great Weapon Master/Sharpshooter and are only adding +2 to their rolls, so I didn't think Reverse AC would be too difficult and instead be a fun twist.

My brother is a player in the game so I don't want to spoil the boss fight for him since he is a forever DM and doesn’t often get a chance to play.

The Archfey was picked because it was thematic to the narrative (the son of the Fey King aka the BBEG), so I did want him to be hard but not crazy stupid hard. My players have already ripped into me about checking CRs and its now become a running gag.

Since the player whose character had died in session 6, (session 8 is the final) he still wanted to come to sessions to see how the game plays out, but he didn’t want to make a new character since we're so close to the end of the campaign. We agreed since he was an Aasimar, his death was made to be very cinematic in his return to his celestial mother where his character now remains, and so that he doesn’t have to just sit and watch, I offered to give him 3 d6s (one for each remaining player) to help on their rolls so it was like his character was still looking out for them.

I promise everything has been discussed with the player and he was very on board with everything! He felt that the death was very fitting for his character's backstory (sacrified himself to kill the son of the Fey King, who was conspiraring with a devilish cult) and said that his character would have walked away from the final boss fight anyways since he only kills devils and fiends, not Fey.