Solutions for Act II Scene 1 - An Ordinary Puzzle

After identifying a few of the less ambiguous terms in this puzzle, a clear trend should emerge: Each entry can be described with a phrase that includes an ordinal ("first", "second", "third", etc.). The title "An Ordinary Puzzle" subtly hints that this is the correct approach. The terms should be determined as follows:

first person
first aid
one third
fourth avenue
second coming
fifth column
second derivative
minor third

first commandment
second cousin
third dimension
third time
fifth Doctor
Beethoven's ninth
first law (of thermodynamics)
fifth position
July fourth

Charles the first
fourth wall
plead the fifth
Family First
third wheel
Seventh-Day Adventist

The next step ought extract letters somehow, and should preferably use the fact we have an ordinal in each phrase. The trick here is to realise the most natural interpretation of each phrase with this in mind is that each ordinal is referencing the letters in the other words making up its phrase. For example, "first person" implies the letter P, while "sixth-formers" implies the letter R. In some cases letters could be counted over multiple words.

Continuing in this way, the letters extracted per column give the words/phrases PRAENOMEN, COMMONALTY, and COLD FEET. Again we want to iterate the previous step on our new phrases, since these alone do not suggest a final answer. Indeed, each term is also a synonym for an "ordinal phrase":

first name
third estate
second thoughts

Indexing in again as before, we get the letters N, T, and H, spelling the (rare) word NTH, a term encompassing any and all ordinals.

The answer is: nth