Can I use past simple instead of past perfect in cases (1) and (2)? If only one tense is possible, please provide some explanation. Thank you in advance.
Neither of us had been to France before, but we knew some French from our time at school and (1) managed (had managed???) to brush up on the basics. Now we were wondering if we had made the right decision. We had planned our route carefully in advance, but we (2) forgot (had forgotten??? ) one important thing: the weather.
The past perfect in a tale can orient the listener or reader to the sequence of events. It is not explicit in the snippet you've quoted, but "had been to France" implies a time-reference for this story —the time of your arrival in France. You "had brushed up on the basics" before your arrival. You "had made the ...decision" to go to France before your arrival. You "had planned your route" before your arrival. And when planning, you "had forgotten" one important thing, the weather.
You could substitute the simple past and allow context alone to orient the reader or listener, and context may be sufficient to prevent confusion. But it might not be. With the past perfect, the time-relationships are clearer.
