[RPG] Do other creatures provide cover

D&D is not a combat simulator

All games make compromises between playability and simulation. Chess, for example, falls on the side of playability while the simulators the Air Force uses to train their pilots fall on the side of simulation. As war games go, D&D is simulation light, playability heavy - tending more towards Chess than an F-16 simulator.

I appreciate that you understand that a creature does not occupy a whole 5-foot square, however, it goes further than this: combat does not actually happen in a series of discrete turns. Its funny to contemplate the fighter, just as the orc raises his scimitar for a killing blow, holding up his hand and saying "Wait, its the wizard's turn". Further, even though a creature is placed inside a 5-foot box for playability reasons its perfectly feasible that over the course of a round they are darting about all over the place in and around that box in order to get the clear shot they need.

As another complication, many spells do not indicate that something physical travels from the caster to the target - are these to be treated differently in your system? Notwithstanding, for those that do, there is no reason to suppose that a magical effect needs to follow physical laws - I can easily imagine the green ray from Disintegrate passing through the bodies of several creatures without harming them on its way to the target. Alternatively, who says that these effects go in a straight line? Maybe they zig-zag around the intervening creatures because of, say, magic?

Physical weapons do not go in straight lines either: they follow a ballistic trajectory. Arrows "arch" which is why we have the word archery - historically missile troops shot over the heads of their melee companions to drop death from above on the enemy - much like modern artillery and air strikes do today.

I judge that this means see clearly, i.e. for long enough to cast the spell.

I disagree, seen is seen but even if I did accept your judgement casting most spells takes 1 action. How long is that, exactly? As long as it takes to swing a sword? Clearly not, because a high-level fighter can swing his sword multiple times in the same time period and move in between. The rules are clear that a round is about 6 seconds, however, there is no RAW for how long a turn or an action is.

However, if you have considered all of the above and still find that it grates on you then you might consider a house rule. These are only worthwhile when they add more to the game than they cost.

Personally, I don't feel the rule adds anything much but that is a judgement call and your judgement is not mine. Here are what I see the costs are:

• It will rarely come into play. The best combats in D&D 5e pit approximately equal numbers against one another: 4 on 4 or 5 on 5 given a typical party. With so few combatants on the grid there is almost always going to be a spot within movement that a ranged attacker can get to so that they have a clear path to the target.
• It will slow down combat. Each ranged character, on both sides, will spend a little bit longer each turn positioning themselves just so to avoid these penalties. Now, I play Advanced Squad Leader and I can happily spend hours working out the exactly right hex to place my anti-tank gun in or minutes considering if I move this squad across that road just here will his MG nest be able to see them (and more to the point, kill them) but that is the game experience I want from ASL - I don't want it from D&D.
• Its verbose. I realize that this is a draft of the rule but it runs to 6 paragraphs and has a largish number of sub-cases and wherefores that the players have to keep track of - all while remembering which spells are affected and which aren't.

This is what I do:

• I use the RAW but I "eyeball" it - if the creature concerned can easily get somewhere (5-10 feet away) where they have a clear path I just assume they do that and don't get the modifier. I just take it as a given that the creature takes their shot at the most advantageous time.
• If the players decide to engage in combat in e.g. a crowded marketplace full of civilians then I just say "No ranged attacks or large AoE, there are too many innocents about". Of course, their enemies might not be so squeamish but that's part of the fun of being a DM.

Divine Sense can't target something behind total cover.

Your instincts are correct.

The rules on cover state:

Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm. A target can benefit from cover only when an attack or other effect originates on the opposite side of the cover.

There are three degrees of cover. [...]

[...]

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

The usage of the word "concealed" here is problematic, but I think the usage here is assuming you have a solid, non-transparent object providing your cover. I think the intent is clear – if an attack or effect would be blocked completely, then the target is behind total cover.

Since the paladin's Divine Sense feature cannot detect through total cover, I think you are in the clear. The sheet of ice acts much like a wall of force, except the ice can be damaged or broken through, and I'd rule that wall of force provides total cover to enemies on the other side, as it "makes the target more difficult to harm."

This ruling is further reinforced by an unofficial tweet by rules designer Jeremy Crawford from April 2016, in response to a question about whether wall of force provides cover:

Cover is a physical obstruction, not necessarily a visual one.

This also serves to show that the use of the word "concealed" in the description of total cover was in error.