What's the word for the reversability of equal and opposite actions?

Solution 1:

Right and left are inverses or complements (in the limited sense that they are "either of two parts or things needed to complete the whole; counterpart."). They are also counterparts.

But this is a little different from what you're asking for in the title.

• Associative is a property when it doesn't matter what order you press them in (e.g., LLRR is the same as RLLR). This is borrowed from the math concept (e.g., 3+2 is the same as 2+3).
• Invertible is a property when it has a counterpart (e.g., Ctrl-Z is invertible with Ctrl-Y, but Ctrl-C is not invertible).
• (Reversible is another way to say invertible.)

You might also consider borrowing terms from set theory (such as L and R form an identity), but that might be too much baggage for a straightforward notion.

Solution 2:

I'd suggest "inverse" or "inverse functions"

adj. Inverted in position, order, or relations; that proceeds in the opposite or reverse direction or order; that begins where something else ends, and ends where the other begins.

Source: OED

For example, the inverse of left is right. Mathematically, the inverse of the derivative is the integral. Etc.

Solution 3:

I would like to suggest counteraction and adjustable.

counteraction noun
a force or influence that makes an opposing force ineffective or less effective

adjustable adjective
capable of being readily changed

Here is how I would use them in your examples,

1. ... because right and left are counteractions.
2. Undo and Redo are counteractions.
3. The length and radius controls are NOT counteractions when used together. Length is adjustable while radius remains fixed and radius is adjustable while length remains fixed, but adjusting either breaks the adjustment of the other one.