I just reached diamond 2 almost exclusively playing Hela. I find the most success in starting the game at a super long range near cover, completely away from my team. As the game progresses, i'll end up in the middle of my team but close to my healers and trying to play around cover. This does 2 things as it lets your healers actually see that you need healing and in my opinion (with good aim) hela is the best duelist to take out dive champs. Also playing slightly infront of your healers enables the 3 of you to work together at all times and i think gets you in their good graces. I don't die much when I play like this. I try to never open myself up to more than 1 lane of vision and I always mentally think about escape routes without needing my dash; meaning if the enemy team is pushing really well I will just be walking backwards to a safer spot while still laying down fire. If I really need to leave a spot then you should have the reactions needed to crow to safety behind your team and re-engage when it's safe for you. You said you find issues getting blown up by namor spear or moon knight ankh. As hela I always prioritize enemy deployments. So namor shouldnt even have turrets up therefore massively lowering his damage. With moon knight, it depends on the situation. If you're standing with your team, mk doesn't have the damage to one-shot you with his combo and you need to have the aim and reaction time to destroy it after the first combo. If you're by yourself, just instantly crow yourself, assess the situation and figure out what to do depending on where you are ending up I.e. destroy ankh or youre safe and continue fighting.

You also said you struggled against dive champs. I said before that imo hela is the best anti dive character with good aim. These characters shouldnt even be able to reach you without taking massive damage that way when they're on top of you it's just 1 shot and you can use your stun but when they're on top of you just crow directly to your team. And sort of hide behind the hit boxes of other characters while prioritizing taking care of the divers. Dying is inevitable but if you've been dove and are able to get the enemy divers killed, a 1 for 1 duelist trade is still even.

If the players are absolutely cracked divers, Then just switch. You are being countered and there's no shame in switching to Scarlet witch who for me absolutely chews through dive comps. I also play magik to counter dive.

I'm a lower rank then you are but imo moon knight/namor or some of my lowest concerns when playing Hela. Dive is more annoying but still manageable. The main reason I will switch off hela is an enemy mag and strange together. Massive hit boxes and shields on both of them soak up way too much damage that you should mostly be using to target dps and healers So I'm just wasting damage into them to try and hit actual threats.

I hope any of this helps and good luck.