Meditating on Grappling (5e + B/X)

Over at B/X Blackrazor, JB ran an adventure for his kids and shared a little house rule he uses with multiple attacks:

My (simple) house rule gives creatures with multiple attacks one attack roll per round versus a maximum number of opponents equal to its multiple attacks...so the [carrion] crawler (for example) would be able to attack up to eight opponents, but regardless it would make only one attack per character per round. This is something I've been doing for a while now, and I find it works well in practice

This got me thinking about how I might express this idea in context of the Advantage/Disadvantage system from 5e (because I really want to see how many applications this mechanic can have). My starting point is an old-standby in the OSR and was even codified in S&W Complete:

Just as shields improve armor class by 1, fighting two-handed grants a +1 to damage rolls . . . and fighting with a weapon in each hand gives a +1 to hit. (Note that fighting with two weapons does not actually give two separate attacks; it just increases the likelihood of landing a successful blow.)

So, multiple attacks don’t result in extra die-rolls, but rather in a bonus to-hit. Why not give advantage instead? Thus, the carrion crawler in JB’s example would get one attack, but would roll two dice and use the higher of the two.

Since we are already going down this rabbit hole, shield-fighting should do the opposite. Rather than affecting AC, having a shield puts attackers at a disadvantage, meaning that they would roll two dice and take the lower of the two results. This advantage would be cancelled out by either multiple attacks or flanking.

Two-handed weapons, therefore, should have advantage on damage rolls.

This all leads to grappling, the bane of every D&D combat ruleset. The reason for this is rather simple: combat in D&D is very abstract, which is why multiple attacks can equal a bonus to-hit (two-weapon wielding in S&W) or an advantage as in my musings above. Grappling, by its very nature is . . . not. Thus, once anyone tries to go down the road of specificity and handle grappling it gets messy.

The above concepts for using advantage/disadvantage suggest a nice and simple abstraction for grappling:

Anyone can grapple. A successful attack roll indicates that grappling is taking place and both the attacker and target are now at a disadvantage. Thus, anyone attacking those currently grappling gets to roll two dice and chose the higher of the two and those grappling must now roll two dice and take the lower of the two if they wish to make an attack. This can only work on creatures that are larger than the attacker at the Referee’s discretion.

This rule could also work for shield bashing, possibly even cancelling out the disadvantage the attacker gets for grappling because of the shield (though they would lose the defensive capabilities of the shield).

I am currently play testing these ideas with a summer-time  campaign with my kids. The first time it came into play was an encounter with a ghoul. Having shields made the encounter survivable and hammered the point that attacking with advantage and a paralyzing touch is just as nasty as three attacks per round with a paralyzing touch.