Now, if all of us could congregate like that someday, would that not be something...

Maybe in Amoy! And we'll all pretend to only speak Hoklo.
On to my Amoy field report. First, my M.O.:
1) Hoklo as default language.
2) Refused to switch to Mandarin with anybody I knew or believed to be a Hoklophone, with one exception.
3) Sometimes pretended to understand, but not speak, Mandarin.
4) Sometimes pretended to neither speak nor understand Mandarin.
5) Generally switched to Mandarin with people who couldn't speak Hoklo AND could be assumed to be making less than about 2500 a month, unless I'd already decided to pretend to be Mandarin-incapable.
6) Assumed everybody spoke Hoklo even if they didn't look it.
7) Allowed myself to be switched to English as part of my no-Mandarin persona. Few takers.

Allowed myself to be switched to Mandarin during certain "crucial interactions", such as getting a haircut. Mostly this was b/c I'm not a complete speaker myself -- there are lots of things I don't know how to say in Hoklo "the way a native speaker would say it", and sometimes I'm just in no position to fight the good fight, 反燕復閩 HAHAHA...
My findings:
1) Small business owners tend to speak Hoklo.
2) People over 50 tend to speak Hoklo. Most in-migrants seem to be younger than 50, or else they came from close-in places like Po·chân or central Hokkian in a day and age when Amoy was "all Hoklo all the time". Don't laugh. According to Lîm Kiànhui, that's how it was right into the early '90s.
3) Much of the Hoklo U here in Amoy is spoken by people from other parts of Banlam. They may even live outside of Amoy, in Ciangciu esp. Much of the Hoklo heard in Amoy is not what scholars call Amoy Hokkien / Amoy Banlamese. We also have to remember that there've been different dialects in Amoy since the 19th cen.
4) People under college age may tend to understand, if not speak, Hoklo. This is my conjecture. Some are migrants' kids, but most are probably from local family. The youngest person to speak Hoklo to me was high school age. This was on the BRT. The bus was packed to the gills. I bulled my way on there airport-bound with two bags and a big suitcase. Kind of selfish, I guess. There was a young family behind me speaking Hoklo with a Ciangciu accent. When they wanted to get off, there was no getting off -- the train was too packed. The man said good-naturedly, "Chuqu yixia ma, chuqu yixia zai shanglai. You ren yao xiache." The kids in the doorway just stood there. I was standing next to them so I yelled, "Āupiah ū lâng beh lo̍hchia--lah! You ren yao xiache la!" No effect, so the family had to just bull their way out. A few stops later, it was my turn. As soon as the doors started to open, I grabbed my suitcase and started bulling my way out, saying, "Ciohkoè, ciohkoè!" And to my surprise the kid who was in my way said, "Hó, hó, hó!"
5) There are young people around 20 or so speaking Hoklo in Amoy. In every case, based on their appearance and the fact that they were speaking Hoklo, my guess was that they came from Ciangciu or Coanciu.
6) Amoyans and Banlamese in general are very sensitive to badly spoken Hoklo. They usually switched me to Mandarin straight away if they caught me stuttering or speaking non-native Hoklo. I could usually switch them back by speaking good Hoklo, but not if they were under 30. This is close kin to my Taiwan experience.
7) Hoklophones in Amoy could understand my Mainstream Taiwanese w/o problems. The first day or so, sometimes I would switch to Amoy Taiwanese pronunciations, but this actually caused problems, I think b/c it slowed me down and wrecked my flow -- by Hoklo standards. Rhythm is a key, or THE key, to cross-dialect Hoklo communication. In Cantonese, U can pause mid-sentence, but in Banlam and Taiwan, thou shalt not pause nor err by a millisecond in the timing of your stops and your vowels, etc. My best bet was to speak the dialect I speak most and speak best.

Attitude is everything.
9) Non-Hoklophone in-migrants like to pretend that they can use their "Mandophone ear" to make out things being said in Hoklo. For example, I told the tea thâukeniû, "Mài lām thn̂g, mài pengkak." (The second half may actually be a MY/SG-only usage.) She said, "Shenme? Ni yao binggan?"
10) White-collar workers tend to be Hoklophobic. I went into a bank -- Ē·mûi gînhâng, no less -- to open an account. The security guard / front desk guy didn't speak Hoklo. I pretended to only partially understand Mandarin, maybe to the extent I'd understand Hakka in real life. At times he communicated with me using hanji.

The tellers both spoke Hoklo. A 45-ish lady teller opened my account -- hō·thâu, not kháucō

. She was so hardwired to speak Mandarin that she would say everything to me in Mandarin first, then repeat herself in Hoklo. I would pretend to partly comprehend the Mandarin part. I think maybe she thought the whole thing was either a test or a jest.
11) By assuming that everybody spoke Hoklo unless proven otherwise, I actually "discovered" Hoklophones I would've guessed weren't, from the looks of them or their station in Amoy life.
12) There's a lot of Amoy/Banlamese vocabulary I don't understand. This mystified me, but now that I think of it, Hoklo conversations in TW tend to be full of references to TWese places and faces. W/o that local knowledge, it's easy to underestimate our comprehension of the language.
13) Amoy is a lot like Singapore, in several ways! Amoy also has the benefit of a stunning physical setting. But the whole place seems to've given itself over to a laid-back, materialistic me-tooism, an Amoy version of the New China dream, light years away from the pride and fury of Canton.
14) At the airport, there were connections to places like Jakarta, Singapore, Manila, etc. See a pattern? Int'l flights, but not domestic flights, were announced in Hoklo alongside Mandarin and English.
More later if I think of anything else.