Background

Duuh! Critical hit with the 20.

You attack again with your bone

Miss! Can't even hit a stretched out rope with ot without proficiency.

Rope swings its pod again at Maggie

Maggie is hit again

Still missing there barbarian...

Maggie is all stuck at this point and completely grappled.

Rope attacks again

Rope hits and does enough damage to kill Maggie

She explodes at death.

Rope coils away from the Mephit enough that it only takes 5 points damage. The rope falls to the ground and lays over the bones. The mimic doesn't appear to be bloodied (taken half of its hit points in damage). It is 10' from you

What do you want to do?

Your attack roll fails to match or exceed the Mimic's AC. You miss.

The higher the AC, the harder to hit. It either hits or doesn't. Your rnd() number must meet the AC. Your skill checks must meet the DC of the skill. Your save against a spell must meet the spellcaster's spell DC. Your stealth check must meet the prey's passive perception. Your strength check must meet his strength check in a contest. This is the d20 system in a nutshell.

It is good to roll for damage too, just in case you hit. I'll allow the transfer damage idea under the rule of cool.

You have one interaction with object, one action, one bonus action, and one reaction per 6 sec round. You can yell at the rope for an interact, attack it as action, but you do not have a bonus action type ability that I know of, and there is nothing to react against.

As an action you can do a variety of things as listed on your character sheet. You cannot do an attack and do a dodge in the same action (there are exceptions, but not here). You do not have the parry reaction (that's a battlemaster fighter thang)

Since this is pbp, I'll assume you are giving me two actions because you plan to do them in sequence.

The rope transforms back into a blob, dropping all the bones and floor that it was stuck to.

You are shocked at this and take the defence action of dodge which gives your attacker disadvantage on its attack.

It transforms into a ballista complete with tethered bolt, cocked back and released. He does not attack this round, so your dodge action goes to waste as you stand there getting ready to dodge.

Since you are trying to interact with a weapon held by an enemy or in this case the monster itself, placing the bone is an attack, not an interaction. Your roll for it higher than its AC, so the rule of cool says you jammed his gun.

Since it cannot act as an object, it had transformed as a loaded AND fired weapon. It takes an attack action. The bone jams and is now stuck. Way cool.

Your turn.

I proceed to attack the Mimic as described before. And I pick up another long bone from the pile of previous victims.