What do each of the actions increase, numerically?

You've got it almost correct; the only difference is that Downvote decreases Users by exactly 0.5. Also, it should be noted that there is another action that appears after graduation: Close. Close does -2 to Questions and +2 to Quality.

One thing that the other posts seemed failed to mention: The decay point reset by voting and decaying aren't equal. It will usually take 2 turns to decay after increasing that stat, but takes 3 turns after it is previously decayed.

Instead of a "decay point", I suspect the stat is not an integer internally, and is rounded when displayed. Each time increasing the stat by voting, it goes to the greater integer than the rounded value. And for each turn, the stats are decreased by somewhere between 0.3 and 1/3 hearts. But the stats also affect each other and I don't know the exact formula. (Constantly decreasing 1/3 * 4 hearts also makes winning impossible, so it cannot be strictly following this.)

So if a state is not full, left alone one turn after last vote, or two turns after last decay, and increasing it will give you 4/3 hearts. The next optimal options are voting what is just voted, or one turn after decay, which is 1 heart. Voting full stats is probably never optimal by this calculation, but I'm not really sure. Also be careful not to let two stats fall into the 4/3 hearts optimal state together, or one of them will decay.

The turns you can ignore warnings seemed decreasing during each period. And the affection of a low question stat to the answers is increasing. I'm not sure whether it is because the stats are once greater, or this still happens if you mostly do nothing, or it is because you have a fixed number of chances ignoring warnings and the affection of questions has a delay. But unless you did something bad deliberately, I think it is pretty safe to ignore them. That is: after the grace period is below 4 days (except downvoting decreases user's by probably less than a day, which is effectively still one day), you won't get any more warnings. And questions never have an visible effect on answers if you are following the right strategy.