4/5

Now, let’s sum up what we found out:
>There’s a mysterious space rock, containing Infinity Energy.
>A power capable of defying death and extending life.
>Which can be also used as a power source.
>It’s related to Key and Mega Stones (probably more powerful).
>Which means it reacts to the inner desires of a trainer-pokemon-pair with a close friendship.

How does this fit into “Ingrained”? Let’s see:
>The meteorite is conveniently stashed inside a bag.
>That bag belongs to a girl, who is experiencing the worst time of her life (soon going to be her last).
>She’s scared to death (pun not intended) and instinctively begging for her life.
>Near that girl is her faithful pokemon, which cannot bear its best friend’s imminent demise.
>Being overwhelmed with the situation, it uses a move capable of healing, even though it cannot prevent death.
>But Roselia doesn’t care, as it’s preoccupied with its desperate wish to protect its trainer.

Add one and one together, sprinkle some “friendship is magic” bullshit over it and voilá:
>Roselia’s Ingrain is now able to defy death.
The meteorite acts as some sort of arcane battery, fueling the roots with Infinity Energy and granting May and Roselia immortality (as long as they’re under the effect of Ingrain, that is). With their bodies being kept alive - no matter how lethal their injuries are - their souls are unable to leave. Severing the roots is futile, as Infinity Energy made them more resilient. So cut them as much as you want, they will immediately grow back. The only way for May and Roselia to escape their fate is being nursed back to full health. Yet this seems impossible, as whenever they get healed, they’re immediately maimed back to the brink of death.

All they can do is endure this agonizing equilibrium between drowning and reanimation, and wait for the meteorite to run out of energy, or hope that someone will eventually find them. Well, as for the latter...