Who to +10?

Camilla or Corrin. Because I feel like they are too underated and I have found a way to make them KILLER.

Marth is great and benefits from increases to all stats.

A +10 Marth with -HP/+Spd and refined weapon
HP 44
Atk 54
Spd 44
Def 36
Res 30

He can tank, give him drive skill/seal and he buffs allies, great counter to dragons.

That being said, I would wait until 3.0 update to see if any units get demoted. You might find someone you really like is no longer 5* exclusive.

My most used units for PvE are my Fae and A!Tiki. A +Atk A!Tiki with BoF will OHKO most red and green units up until the hardest difficulties and can easily out-trade just about every non-blue unit. She also tends to do fine against lances most of the time.

Back in the days I started yolo-merging some Ravens until I +10'd him when he got his prf. Now he is one of my favourite units to use in arena since I recently gave him G Duel Infantry 3. With Galeforce + Heavy/ Flashing Blade Seal he is really fun to use and has the same Arena Scoring as LA!Hector or Zelgius, which is pretty amazing! You could give him Infantry Pulse to benefit more from the +5 HP from G Duel Infantry 3, too.

Roys our boy!

I feel pretty confident about what I'm about to recommend.
- don't merge dancers unless they're playing enemy phase (ex: some kind of warding breath glacies on Silvia maybe? She's quite fast and will often double naturally at +10. But it probably won't be enough on abyssal where they have absurd amounts of HP and you really need 50+ atk and sometimes triangle advantage to get through them in two hits). They won't be fighting much and they do their job just fine without merges, so your feathers aren't getting good use.
- Nowi is extremely solid in PvE. I have her +spd -res (waiting for -HP) using lightning breath+spd, 5 merges and summoner supported (so she's like a +10 with 3 extra HP). Hard to double and even harder to take down. She'll often tank hits from 3+ units in one round if a maximum of one is green and none of them is the boss. She also has a tendency to recover her QR even if she gets knocked out of it, or if she can't recover it, at least she's exchanging hits one for one rather than getting doubled like the -spd variant people tend to recommend for arena. The main problem I was thinking about today is that L.Tiki+0 is very similar to +spd Nowi+8 with arguably a better stat spread, a killer C-skill, and a DC weapon without cooldown penalty which enables her to do something like special spiral + flashing blade + aether and still use her A-skill for extra atk and spd. Nowi can do a similar thing using light breath and atk smoke, but I think L.Tiki might have her outclassed.
- Felicia can be powerful with merges, but I wouldn't consider her ideal for PvE. Her atk is low, which can be a massive liability on abyssal maps where one of the challenges is the sheer amount of HP to eat through. I have her at +6 neutral. I got through Hector abyssal using flashing blade glacies, and I was stacking on it hone atk from Nowi, ally support with Nowi, Marth's Falchion +2, and drive atk (1 or 2, I forget). I also got through L.Tiki on infernal with a desperation glacies, just barely.
Generally, if you're already using Veronica, I wouldn't put Felicia on the team also. Use someone with higher atk. Felicia's designed to increase her damage by hitting people where it hurts, but abyssal map units often have 30 mixed bulk and 80HP which basically requires her to have 55-ish atk and activate glacies, which takes a lot of investment.
- more to come in a moment.

Prelude to my second half: if you read what I wrote at the link below, I explain multiple reasons why I think player phase is going to gain in importance in the next few months. https://fireemblem.gamepress.gg/q-a/some-comments-direction-game-and-meta
That said, I tend to believe that tanks, especially mixed tanks or fast tanks (and above all, mixed fast tanks like +spd Nowi) need and benefit from merges more than most other units.

Now for some of the candidates you named:
- Eirika has the potential to be a terror if she's sharing +6 visible buffs with teammates (for +12 to her, because she receives the buffs once directly, and then her weapon copies them for an extra +6), and tactics do have the range that allow you the maneuverability that abyssal maps demand. On +atk, she reaches 62 atk without additional skills.
- Raven is a strong candidate, especially with galeforce potential. Similarly for Cherche. Remember, player phase sledge hammer is a good strategy for abyssal.
- Kagero is outclassed by Sothe at the moment. Unless you're expecting a prf or want to save L&D fodder, I wouldn't choose her over Sothe.
- A.Tiki I don't recommend because she's slow and gets doubled a lot, which is not generally a good thing to begin with because PvE has plenty of mages and she can't wall hard on def if she wants to invest in her res, so she's generally just taking a lot of hits and losing her QR easily.
- Roy isn't quite tanky enough for my liking without close or distant def, and if you ask me, he gets doubled too much and doesn't fire back enough damage. I'm not saying this from experience though.
- Merric: atk is just a little low for abyssal maps. Like Felicia, it's possible to work with it, but I'd sooner just grab a blade tome.
- Cordelia vs Shigure is an interesting discussion. For PvE, I think prefer Cordelia because she will be fast enough with merges and the extra atk is becoming more and more useful

My final notes:
- A lot of units can actually fight very well in PvE with few to no merges. Blade mages, B.Veronica, L.Marth, special spiral Ophelia just to name a few. These are units that I consider to have strong potential in abyssal maps with no merges.
- A couple of people suggested Marth. If you're using him as a healer, I probably wouldn't bother with the merges. Falchion+eff + renewal + dual drives doesn't gain any power between +0 and +10. Same for Titania, except she's a very potent mixed tank if she's not using the prf (or if you're willing to do the obtuse and use CA) so if she switches roles then maybe yes, even in my books.
- I tend not to like to merge more than once on units that use mainly just one stat (ex: non-spd builds of Lilina) or units that gain most of their power from buffs (ex: blade tome mages). Eirika and L.Marth are the exceptions, because they actually can use all 4 stats, especially with the buffs and merges, to become devastating monsters.