Three-part question

1. Honestly it doesn't matter. Chloe is enough to take care of anything you'd need an archer for. Level him if you like him or want to use him, otherwise save your embers. He's a good crit servant, but unless you have a 2030 or 2 lying around, or plan on putting him in a team with Jack/Fuma, you likely won't be able to use him to his full potential.

2. I've never gotten Fionn before.

3. Personally I wouldn't burn him unless I have at least 1 more of him, but if you don't care about him at all then go ahead. He doesn't do anything worth keeping him around for, and as far as lancers go Cu is much better than him.

1- As my only Archer above 3 stars, I'd say he's a great investment, specially after those upgrades since they undoubtely made him one of the best SR servants in the game. I was skeptical about leveling him at first, but man, did he shut my mouth. His NP damage isn't that impressive to be honest, but given how it ignores defense it can prove quite useful, and along with his newly upgraded skills, he can cause some massive damage through crits I highly recommend leveling him even though you have Chloe and Atalante.

2- He was one of my first SR servants, but I never really used him. I've never seen anyone saying anything even remotely positive about him, and I never really felt like finding out for myself if he was that bad. So yeah, he's been gathering dust for quite some time and will stay that way for a long time.

3- I personally haven't burned a single SR servant or higher yet, because I like to collect them even if I'm never going to use them. If you're aiming for something on the rare prism shop, then go for it. It's your call.

1 ). Emiya is a Tough Crit damager, some servant with high damage only have steroid on their kit ( Glass Cannon/Fragile Striker ) but Emiya have more balanced build in him.
He is worth the Investment, even if you got 2 maxed archer he can be dependable to use on various situation.

2 ). I have Fionn on another acc, but not on my main. Not a really solid Lancer, but he is still usable as wave clearer and daily Farmer.
I got him while rolling for Brynhildr last year ( Fionn spook is one of a real horror to me ).
Rarely use him and i better use my Brynhildr for daily farm.
For more Memes, i have a bad idea by using Bryn skill 3 to Fionn ( Equipped with Crit CEs ).
Totally not cool : (

3 ). No matter how bad be is, don't burn him.
And if you burn him, better becareful he like to spook people.
Spooked by him on your favorite servant banner is painful.

I would say atlante is very different from emiya. For chloe, her kit is overall better. However, there will be times you want more than one archer in a team so raise emiya if you have spare embers.
I also have fionn who has been on standby since last year? I think. Despite lacking lancers, i still think he would be better off as a rare prism. I will wait until i get another good lancer though.

1) You shouldn't, rather...would you like to? He's a strong archer but he needs a lot of investment like completing all his interludes, buff quest, leveling his Hakweye to 10 and he asks for troublesome mats (Too many void dust).

2&3) Yes