They fall.
A spell's entry, plus the general rules for spells, together give a complete description of how it changes reality. AD&D spells tend to be very open to creative use, by extrapolating logical consequences from the effects a spell describes, but there's no rules or tradition support for extrapolating extra reality-changing effects than those which are described. Where a spell is silent on how it works, it's up to DM interpretation; but where a spell is silent about what it does, it simply does nothing.
So. The spell levitate has a maximum weight limit, and describes no slow-falling effect while exceeding it. Since "falling slowly" is a magical effect, and the spell does not describe it as having such a magical effect, it doesn't. And since it has a weight limit, that is how much weight it can lift. Without adding any new magical effects than those described, there's simply no provision for the spell to have any effect at all while its stated limits are exceeded.
However, it wouldn't "break", as the spell also makes no mention of being dispelled by exceeding the weight limit. It would just fail to be able to levitate you, until the excess weight is removed.
(Of course, a DM can change or add to spells as they see fit, but that should go without saying, and besides which doesn't change what a spell does absent any after-the-fact modifications a particular DM might make.)
As an aside, D&D doesn't simulate weight-as-force except in the damage rules (where it does not simulate it so much as nod in the general direction of our physical expectations). D&D generally simplifies weight to a pre-Newtonian concept of weight-as-mass. You can ignore added force due to the acceleration of falling in most cases, since the game systems don't benefit from the extra calculus and may actually suffer added weirdness if done.
From page 11 of the Monster Manual:
A creature that can make multiple attacks on its turn has the Multiattack ability.
So Multiattack is an action that allows a monster to make multiple attacks; that's the whole point of it.
If you look at any monster that has Multiattack, such as the Aboleth (first monster in the book with Multiattack) its attacks will be listed individually, and then Multiattack will say something like:
Multiattack. The aboleth makes three tentacle attacks.
So even without the general rule, the monster's statblock usually specifically states that Multiattack is made up of multiple attacks.
This being the case, the Rogue can only Uncanny Dodge against a single attack, but on the other hand, the creature has to make a Wisdom saving throw every time it wants to attack someone protected by Sanctuary.
Best Answer
A creature affected by slow makes only 1 attack during its turn even if they have multiattack
The slow spell description states that:
There is not much room for interpretation, it is clearly spelled out. The creature gets one ranged or melee attack no matter what. In the case of a hydra this makes it do only a single attack per turn.
Do note that this does not stop the creature from doing more attacks during a round as long as they are done outside its own turn since slow only limits the number during the slowed creature's own turn.
This means that a creature using legendary actions (at the end of other creature's turns) is able to do the normal number of attacks granted by those legendary actions regardless of slow.