A creature can be grappled by multiple creatures, and it has to make a separate escape attempt against each one.
Grappled
A grappled creature’s speed becomes 0, and it can’t
benefit from any bonus to its speed.
The condition ends if the grappler is incapacitated
(see the condition).
The condition also ends if an effect removes the
grappled creature from the reach of the grappler or
grappling effect, such as when a creature is hurled
away by the
thunderwave
spell.
There's nothing in the Grappled condition that would prevent multiple creatures from grappling a single target.
Where it gets complicated, though, is breaking free. From the grapple rules for monsters, we have this:
A creature grappled by the monster can use its action to try
to escape. To do so, it must succeed on a Strength (Athletics)
or Dexterity (Acrobatics) check against the escape DC in the
monster’s stat block. If no escape DC is given, assume the
DC is 10 + the monster’s Strength (Athletics) modifier.
This says that the creature must succeed on a check against the escape DC in the monster's stat block. That indicates that it's talking about a single escape DC, a single monster, and therefore, a single escape attempt.
So, this makes it clear how to deal with different creatures with different escape DCs - the grappled creature has to make a separate escape attempt for each one.
All the creatures you've listed have grapples that work exactly the same way, just with slightly different numbers. So, as above, the grappled creature would have to make a separate escape attempt against each escape DC.
This is a fairly powerful tactic at low levels, but against stronger creatures the initial grapples are unlikely to succeed, and higher level enemies often have ways of dealing with grapplers (like teleporting out, or just killing them all at once). It's also important to remember that you can't grapple a creature more than one size category larger than yourself, so your snakes will never be able to grapple a tarrasque (for example).
With all that said, if this tactic is becoming a problem, the DM might consider allowing a monster to burst out of multiple grapples at once, especially if it's clearly strong enough to do so.
No, it's not.
When lifting the other character you are either taking an action, or you interacting with an object -- depending on your DM's rulings.
If the DM rules that the pile driver is an attack (which I'd argue it is), that would cost you an action, and you can only repeat it if you have movement and an action to spend on it. This assumes you've grappled them, and have an attack left.
If the DM rules you are use an object. If it wasn't an enemy, you'd only be allowed to pick it up once. Once you drop it (which is free), it is no longer a free action to pick it up again. Use An Object action:
You normally interact with an object while doing something else, such
as when you draw a sword as part of an attack. When an object requires
your action for its use, you take the Use an Object action. This
action is also useful when you want to interact with more than one
object on your turn. (PHB 193)
Since you can't lift someone and do something else, it might take a full action to lift someone above your head. Remember that a round is 6 seconds, dropping someone more than once (or twice if you have the actions for it) would take about that long. Moreover, really can't lift someone and jump while doing something else, so it could be argued it can't be an interaction and must be a "Use Object" action.
To rule otherwise would allow a free 5d6 damage without using any action, and that, while awesome the first time (and a DM may allow it once, by Rule of Cool), is game breaking.
Best Answer
By RAW, probably.
Here's the relevant rule (PHB, p. 290):
For example, if a creature had the paralyzed condition from both a hold person spell and from the effect of a ghoul's attack, saving against the hold person effect wouldn't end the paralyzed condition until the ghoul's effect was also saved against.
So as long as your DM agrees that your two grapples are separate 'effects', there's nothing in the rules that seems to prevent you grappling the same creature twice. The creature would need to break each grapple separately (per the rules on PHB. 195 for "Escaping a grapple").
However ...
Your DM might quite reasonably rule that, absent other rules, a grappling character (rather than a grappling limb of a character) constitutes a single 'effect' for the purpose of imposing the grappled condition. If so, then you can only impose one grapple on a given creature.