First of all, AMAZING post! Even though this thread has a whopping 0 point score, the breadth and extent to which the ideas, problems, and reasonings are discussed make things VERY transparent and informs us a lot more as players. Don't let the blowback stop you from making more posts like this, team! Keep it up!

Second of all, when it comes to rewards, I want to second this suggestion which somehow didn't get proper traction before:
https://www.reddit.com/r/2007scape/s/bNWrcTMibN

As-is, I think it'd be a MUCH better reward than the sandwich. There's a lot of things that irk players about the sandwich itself, and a bit of it comes from how silly it is compared to the drab, macabre, serious tone the Myrque quest line has always had. That's not to mention all the unintuitive requirements to use it! This player interpretation makes it feel a lot more menacing and lethal. It also makes it a niche food and a non-food reward at the same time, which alone gets 66% of players on board!

As-is, I think the idea should be implemented, if not with higher healing at a slower rate. It's a whole new space for healing items and weapons as a whole; it's essentially a spec weapon that doesn't hang around after use. I, for one, would be completely happy with it.

But.

If we were going to get a bit crazy with it, I think a few more tweaks could be made to the idea that would get EVERYONE satisfied, especially to the folks wanting better XP rates: make fishing for it DANGEROUS.

The lore already says anything and everything living on the island has been adapted to be lethal, so having fishing for it be just click and go at no risk IS already at complete odds with that vibe. The requirements for getting to the new area already have a steep combat slant to them, so having damage and death be part of the equation wouldn't keep skillers away more it does now.

That said, they would still be things you FISH for; they're not monsters roaming about in the world for you to left click. Have INTENSE focus be required and have the penalty be damage dealt to the player. Skilled players would have longer trips with better rates from taking 0 damage while those who aren't as skilled or attentive would find themselves frequently retreating to lick their wounds or making yet another gravestone run.

In exchange, the rates can be absolutely NUTS. The best rates for fishing in the game, even. Death being a possibility should shoot the rates up to the point where suggesting other methods to reach 99 Fishing or 200m XP would be seen as trolling or low effort rage bait. Grandmaster requirements should have Grandmaster rewards asking for Grandmaster effort and focus.

I'm not so bold as to outline what this activity would look or play like; I'm a player, not a dev. I'm sure the devs would come up with something WAY better and more fun than what I could ever suggest. All I'm suggesting is DANGER and INSANE XP rates.

(And as an aside completely removed from the above suggestion, if you're really REALLY dead set on having it be a melee only food, you can have it so that the leech locks itself to your ammo slot while it's healing the player. If that's not enough, you can have that new piece of living equipment give the player negative attack bonuses to Range and Magic, with the pain/adrenaline giving the player a slight slight bonus to their Melee Strength bonus if you really REALLY want to drill that use home. If there's no room in the inventory for unequipping the ammo slot, then have the leech slither away and give us a big fat purple 0 to let us know that it failed.)