How do I adjudicate the logic modality of certain death regarding the Fast Friends spell?

When I cast the Fast Friends spell could I assign the afflicted target to a task that leads to a die roll that has a chance of 98% from death by falling damage without the spell ending immediately?

If the activity would result in certain death for the creature, the spell ends.

Does the task have to necessarily lead to death for the spell ending condition to take place?

How do I adjudicate death probabilities for the purpose of the duration of the Fast Friends spell?

Skills with different abilities: How to adjudicate what combination to use?

In the D&D 5e Player’s Handbook, there is a variant option called Skills with Different Abilities:

Normally, your proficiency in a skill applies only to a specific kind of ability check. Proficiency in Athletics, for example, usually applies to Strength checks. In some situations, though, your proficiency might reasonably apply to a different kind of check. In such cases, the DM might ask for a check using an unusual combination of ability and skill, or you might ask your DM if you can apply a proficiency to a different check.

Essentially, the rules allow for a DM to ask for an ability check but the player can apply a skill, normally associated with a different ability, to the check.

For example, attempting to blend into a crowd by engaging in conversation and acting like you’re meant to be there might call for a Charisma (Stealth) check. So the player would roll a Charisma check and apply their Stealth proficiency – if they have one – to the check.

However, not everything is clear cut, there are often overlapping skills which could equally apply. To use the same example, one could argue that it is not a Charisma (Stealth) check, instead it is a Charisma (Deception) as you’re deceiving the people about your intentions or (Persuasion) as you’re persuading them you’re meant to be there.

Another example, a player wants to parkour their way up to the top of some buildings. One could equally argue that this a Strength (Athletics) check as that is the usual ability associated with climbing or that it is a Dexterity (Athletics) check as you have to be exceptionally precise to quickly grab small ledges so you don’t fall to the ground.

So, my question is, when a player describes an action they want to take, how do you adjudicate which skill and ability to combine to make the check?

Answers must be backed up with your experiences of using this variant rule

Firing a relativistic kill missile at a god in an otherwise medieval world: How to Adjudicate? [on hold]

My players have come up with a (deviously clever) solution to an impossible quest to kill a particular god-like being: accelerate an unobtanium missile at it to relativistic speeds and smash it into said god. They have the missile in question and a method to accelerate it.

My trouble is this: how do I adjudicate a system for calculating the trajectory of such a missile in an otherwise medieval world? I don’t want to say

  • “It hits, you calculate correctly.”

  • “It misses, there’s no way you can do that calculation.” (While this might be realistic, they presumably could get access to instruments precise enough to target a missile like this)

  • “Give me your relativistic equations and I’ll error check them” (This would be no fun for anyone, and I’d be in way over my head)

  • “Make an attack roll/ability roll” (Doesn’t give the thing the weight it needs, and is a bad ad-hoc solution anyhow)

My goal is this: provide a ruling for calculating such a missile’s trajectory (end result it hits or misses) that isn’t overly reductive or as complex as the real thing it’s representing.

To reduce the scope of this question from the theoretically infinite rulings that could be made to something answerable with good subjective, bad subjective: What kind or ruling/system can I make that can engage all the players and give them a chance to affect the hit/miss outcome in a way driven by player interaction? (only a random roll or a character build option is undesriable) How do I give this the weight it deserves without devolving into mind-numbing calculation?

How can I adjudicate one character attemping to jump off a party member’s shield at the enemy? [on hold]

Last session, two of the members from my table attempted to make a combo together! I love when people contribute to the story, especially if it’s going to be something epic like this situation, but I need to learn how to deal with these situations and do the proper checks, because I want them to keep happening.

The situation:

There was a tall enemy, the elf rogue was 20 ft away, and the dragonborn fighter was between the rogue and the enemy. The rogue started running towards the fighter, the fighter held the shield making a step that the rogue could use to jump, the rogue jumped using the fighter’s shield and hit the enemy in the face.

What kind of check should I call to see if the characters manage to pull off their combo maneuver?

At the time I called for a DC 10 Dexterity (Acrobatics) check for the rogue, and a DC 10 Constitution check for the fighter.

If the rogue’s weight is 90 lbs., how much Strength or Constitution should a fighter roll to be able to hold the rogue’s weight?
1 per 10lb => 90lb = 9?

Similarly, if that rogue is fainted on the ground, what check should the fighter do to see if he can carry the rogue?

How far can a character jump?
How far can a character jump getting this type of boost? (I believe that next time I put a broken bridge on the map, they are going to try it.)