A few quick notes: your 2e PDF for the mycoloth doesn't appear to be working on the website. And shouldn't all these plants be vulnerable to fire?
Making saprolings super vanilla feels like a big waste. Even weak creatures can be pretty deep in terms of their abilities. What about an ability that lets a bigger plant or fungus creature eat them to restore health?
You have some good concepts, but I think the whole growth point thing is too clunky to be entirely fun. It's a lot of extra bookkeeping, and it really only comes into play if the mycoloth is killing and consuming multiple creatures in a combat, which, if it's going against PC's, should not be the case. For your consideration, here's the basics of how I'd design these monsters for 2e:
Mycoloth (Creature 6)
* Neutral Huge Fungus
* Trained in Athletics and Survival
* Weakness to fire
* Consume Rot is an activity that allows the mycoloth to fully digest the flesh of a dead creature over the course of one hour. In doing so, it regains a large amount of Hit Points and refreshes its daily use of Sprout Saproling.
* Melee: fist with reach 10 and grab.
* Sprout Saproling is a three-action ability that spawns 2d4 saprolings with the minion trait. Can command all saprolings using a single action with the concentrate trait. Frequency is once per day.
* Swallow Whole up to large-sized creature it has grabbed, deals acid damage.
Saproling (Creature -1)
* Neutral Tiny Fungus Plant
* Trained in Athletics and Stealth (bonus in forest or jungle)
* Weakness to fire
* Melee: vine with agile and grab
* Rooting Mycelium is an ability that changes the saproling's grab. A creature grabbed by a saproling is not immobilized, but must make a Fortitude save at the end of its turn to avoid taking damage as the saproling sap their energy. Play up the theme of a whole swarm draining the life out of their victim.
Personally it feels to disingenuous to the original card in my eyes at that point. But I understand where you're coming from. I was struggling for a time to figure out how to work its 'devour' ability in there, but the devourer featured in 'Bestiary 2' with its soul points made me think that I could bring the growth points from 1e over.
I think you hit the nail on the head by including abilities that let it eat other creatures, but you don't have to interpret devour as meaning it has to get bigger when it eats something. It's a lot easier to just make it naturally big and give it other benefits for eating things.