I'm not a teacher ... native American English speaker ...
"They" is the subject.
"recommended" is the verb.
"waiting" is a noun - created by the gerund of the verb 'to wait'.
"until I return" is an adjectival clause describing the noun 'waiting'.
As a dependent clause, it can have a different tense from the main subject/verb.
The other two examples - I would consider non-standard. The main clause of the sentence has two verbs: "recommended" and "to wait". I would probably use this construction instead:
So, it was recommended we (or I) wait until equilibrium has been established.
My son's pediatrician recommended waiting until he is 2 or 3 before seeing an allergist.
And, yes ... the use of 'is' or 'was' would depend on whether the speaker was explaining why something is being delayed now - as opposed to something that was delayed in the past.
If the son is only a year old, then: My son's pediatrician recommended waiting until he is 2 or 3 before seeing an allergist.
But, if the son is 10 years old - then this obviously happened in the past - so the proper sentence would be: My son's pediatrician recommended waiting until he was 2 or 3 before he saw an allergist.
Student or Learner