Reference Time, Coordinating Conjunctions and Past Simple/Perfect Tense

This is an exercise from Face2Face English course with the solution provided in the workbook, but with no reasoning.

In the exercise below I would like to know (actually confirm mine) the Reference Time, and if there is any level of relationship between coordinating conjunctions and the sentence tense. For example, if, under some circumstances, the tense in the sentence before and after "and/but" is the same when there are no other clues such as time expressions.

I finally finished tidying about 3 a.m. and a few minutes later I was/had been fast asleep. I woke up suddenly at 8 a.m. I arranged/had arranged to meet my parents at the airport at 8.30 a.m. but I didn't set/hadn't set the alarm!

To be specific, do you put both arrange and set in the same tense, or one of them in the past perfect and the other in the past simple (the course provides the answers but I want to know why, and know other speakers' opinions (esp. natives) other than the authors - sometimes there is more room for differences and I want to to know that?

-Usually, the arrangement is done before setting the alarm, but do you treat them as indefinite events with no order before the reference time? what is the reference time in the text (this one is not given, but I can think of one)?

Note: I could have written the exercise with just the answers and asked for the reasoning only, but I think it is better not to influence your answers by the author's. I could be wrong, but what I really want is to understand why.

Answer

I finished ... I was fast asleep ... I woke up at 8 a.m. are ordinary narrative past forms in which RT moves forward with succeeding events.

When you hit 8 a.m., however, you find you must allude to events which occurred before RT, your arrangement with your parents and your failure to set the alarm. These two prior events—or this prior event and prior non-event!—give rise to the distressing state in which you found yourself at RT, 8 a.m. Consequently you should employ the past perfect:

I had arranged to meet my parents at the airport at 8:30 a.m. but I hadn't set the alarm!

This does not express any particular chronological sequence in the prior events because it does not 'express' the prior events per se: it alludes to the prior events as causes of your current (i.e. at RT = 8 a.m.) state of having to scramble to make your appointment.