The formula is simply:
$$
\frac{\mathrm{ability} - 10}{2}
$$
with the result rounded down, if it contains any halves.
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
An Intelligence Score of 0 or 1 gives a modifier of -5.
We know every even number is a + or - depending on which way above or below the base of 10 we go, so it would look like this:
\begin{array}{|c|c|} \text{Score} & \hline \text{Modifier} \\ \hline 20 &+5 \\\hline 18-19&+4\\\hline 16-17&+3\\\hline 14-15&+2\\\hline 12-13&+1\\\hline 10-11&0\\\hline 8-9&-1\\\hline 6-7&-2\\\hline 4-5&-3\\\hline 2-3&-4\\\hline 0-1&-5\\\hline \end{array}
Note that -6 does not happen unless the score somehow drops to a number below 0. If we follow the chart lower, then -1 or -2 would give a -6 modifier, -3 or -4 would give a -7 modifier, and so forth. Same with scores above 21. 22 and 23 grant a +6 modifier. Your judgement call was in error, but that's why it's a DM judgement call. Sometimes the answers do not readily present themselves. No harm done, and now you know better.