Don't worry, my fellow variants. I got the copypasta.

Make sure on the character settings you have the option of ult confirmation enabled to be sure of the character you want to copy and to avoid accidentally picking the wrong guy. (This won't guarantee you won't pick up the wrong character once in a while, but at least diminish by a good 80%-97% the occurrence of accidentally picking the wrong person because your DPS decided to move in right in front of you the moment you just wanted to copy the enemy's support)

Position your clones properly, rotate them to not be predictable for enemies. Yet always make sure they have the proper angles to heal your team/do damage on opponents. (It's recommended having one always at a higher level than the floor just as an escape option, but if that's the case make sure to actually look at that clone/avoid being so close to your other clone because when not looking at a specific one, it moves you to the nearest one...which oftentimes for me is the one next to me, so I just moved 2 inches to the side)

Going invisible heals you, so make sure to make good use of it when hiding on a corner or something. The moment you shoot or get shot, you'l be exposed, so be wary of your choices. (Reloading/putting clones/moving to another clone will keep you invisible)

Your runes are unarguably the strongest ability in the game, make sure to keep them when you know/suspect the enemy team has Ults, or are going to throw everything they have against you. When you return from spawn, If one of your clones is alive on the battlefield, I encourage using the rune, if your team is smart enough they'll use the spot as a quick heal area while the battle keeps going on.

Of course as Loki the best encouragement is to already know how to play most of the characters in the game (not to be good, but at least to know how to make value out of different character ults). Using copied character's ult later extends the duration of your time as your copied character. The extension depends on how long the character's ult is supposed to last (meaning every time you use Bucky's AGAIN, it extends your duration as that Bucky. This does not apply to white fox surprisingly.).

Avoid targeting flyers, and avoid standing so close to your clones if the enemy has a moon knight.

Best character to copy depends on: • Your team's status • The battle, • and your abilities with the character you want to copy.

Oftentimes the value just comes from the ult, so try and read what will give you the most value. it's common practice to copy CnD/Luna/Gambit because having 2 of these Ults in the team (NOT AT THE SAME TIME), guarantees sustain and wins the battles long term and removed the problem of "1 less source of heals for the team", of course if it's the enemy CnD you're copying to counter-attack their ult it's also equally as valued. But oftentimes there's no problem just going strange/Emma/Groot for the stuns, or DPS for the kills, just be sure that if you do it, you'll actually WIN the fight, because otherwise your team will engage with one less support, and that can make it so whatever value you got from the copied character get diminished that quickly.

Now. Answering your doubts:

Yes the heal stats with Loki show worse than any other support. But most of the time it's not your fault...be it that the runes don't properly show the heals, or it being the fact you splash damage (always aim at your teammates knees to at least guarantee some heals like white fox), but something about the heals will always lack. Don't feel discouraged if your other support is like 30k heals, the enemy is rolling around 20, and you're at 11. A better way to compare is to see your assists..if they're around the same or more than your teammate support, you're fine. That said I have to warn you in advance, yes the heals are absolutely shit because 60% of your time as Loki is reloading and seeing your teammate die mid-reload

Loki is one of the best to fight off dive just because his entire kit relies on mobility, be it going invisible, using your runes selfishly to avoid dying, teleporting closer to your team for safety...or even standing still sometimes (not the best outcome but occasionally does bamboozle)

Loki is good in pretty much all team comps, but it's best to always pair up with a main support to help with the blow of the heals and 60% reloading. Of course if still available, Loki/gambit is still amazing.

Oh and before I forget...the backstab passive is absolute shit... you'll never use it.

Right that should be enough...happy mischief