Hello, first thanks for having translated this awesome game!

So, I'm wondering about the Artifacts, can you get Artifacts for characters that aren't on your team or it works like their unique accessory? I'm on my 2nd playthrough (first NG+) and I got Lily's one (the one in the chest in Advent Bay) even if I don't have her, but I can't seem to get the Evelyn's one (from Seraphim). Since it's the only one I miss in Advent Bay and some seem pretty hard to get I want to be sure I can't get it for now, before doing the next dungeon. Can someone enlighten me?

Also I think it should be noted in the "Useful Lists/Tips/Whatnot" topic that you can't get them on the first run in the 90+ dungeon (better I don't tell you how much time I spent reloading in order to get Mylen's one in the first run)...

Yes, you can get the artifacts for characters you haven't recruited.

Conversely, you can't get unique accessories (like Aht's Swift Anklet) for character's you haven't recruited.

From what I've read, it is possible to get unique Accessories for characters you haven't recruited. Considering you can get the artifacts, I have trouble believing you can't.
As for Evelyn's specifically, I know it is possible as I have gotten it. Though I did do the quests for that, she clearly wasn't in my party at the time.

I also can't get the recipe for Sharty's artifact. I cleared Valkyrie Falls (all objectives) and I've 250+ sense but I can't find it anywhere. Is that all you need to do? Or do you need to clear all 90+ dungeons?

I can confirm you can get Artifacts without having the characters in party, at least if you've done their quests. Not too sure if it applies if you haven't done their quests to begin with and hadn't them in your party at some point (in Shipwreck Beach/Sealed Darkness)...

As Reid asked, what's the condition for Sharty's Artifact? And Hannah's? It isn't the lvl90 +NG dungeons (all cleared). Or Workshop statistics (Knowledge/Tools/Sense* 286/127/263).
Or do they require further progression? (Beginning of ch.9, no pending quests, some +NG levels/bonuses left in Yuidora Mine, some Arena battles left(will need Wil to transmigrate and strengthen up first before trying Jane's quests...), next event at mansion will open Secret Seal in Yuidora Mine/Meister unlock (Yuela route).

*It's mentioned in that you have to Clear Valkyrie Falls AND have 250 sense to get Sharty's artifact. But the statistics above indicate it needs something else too. Sharty itself is lvl99, with weapon skill M. As is Wil. And every character without Fountain of Youth treatment (transmigrated ones are ~lvl60 atm).
In case it needs them first, what are the materials required for those two Artifacts?

Pretty sure Sharty's/Hannah's requires Meister/Magnus, I was at end game w/ Emi locked out of those ranks, I had every character (except 5, including those 2) transmigrated before I decided I'd finally start the NG. I also had every artifact.
As for Char. specific accessories, I'll have to confirm this. From what you said, you had clone Gaptahl and couldn't get it, so that makes it easier to check, keeping my items and party from a NG+ will help a lot. Will probably attempt w/ Aht's or something.

edit: And apparently my claim or rather, the claim I saw someone else post, is incorrect. I killed about 6 Tainted Earthmen while having a drop rate of 4 (Ful's Tear + Mill + Base Drop Rate) after putting Aht into my party and got the anklet. After over 50 kills with the same drop rate without Aht in my party, I didn't get jack. Just for extra certainty, I killed over 50 Bekoritos and didn't get the Lucky Bracelet either, I didn't even bother checking if I could get it w/ Sharty, but I'm sure it wouldn't have been hard. Anyways, 200 drops w/o any accessory on both monsters.
edit 2: Meh, since it only took seconds to get her in my party, I decided to check it. 12 kills after Sharty in party to get the bracelet.
I'd say, if you don't have any Ful's Mills or Tears, be prepared to kill about 60 or so of whatever monster you want whatever to drop from.

edit 3: And I decided to test Seraphims. After at least 50 or so kills (that I actually counted, I'd say around 70), with a drop rate of 4, I didn't get a single drop for Evelyn's artifact. I haven't done her quest so my guess is that you have to do their quests. The weirdest part about it is that I managed to randomly get Asmo's artifact, even though I haven't even done most of her quests (only the Sin Gate so far). I'm going to just assume that Evelyn's, Lyphia's, and Roka's are special in this way that you must do all their respective quests. I'm totally stumped if you have done the quests for Evelyn and Lyphia though.

Just a tip; A transmigrated and leveled to almost full stats Mylen, combined with the right weapon and Ful's Mil/Tear, makes gathering the rest of the artifacts a lot easier, with her insane Luck/Move stats and range 9 attack skill(Explosive Barrage).

Lily can also be useful, though she lacks long-range attacks and her Move isn't as good. But she does have Luck.

She was also the first that I got to 99 and transmigrated. I also got her artifact first. Very good char .