OK - I only read about 11 pages of this, so I apologies in advance if you have to repeat. A lot of this could be overkill, but I'm taking a "doctor at a loss" approach and perscribing broad spectrum brewing antibiotics. Some of the stuff here wasn't covered in what I read, while others are restating. My biggest suspicions are as follows #'s 1,2,5,6, and 10 below. Any one of these could contribute
Here are the areas I could see a potential problem.
#1 - Bad/Stale LME
Not sure if this would actually cause your problem, but weird sweetness or stale flavor can come from old liquid extract. Given you're on an island and they probably don't refrigerate the extract on the boat over, that could be a problem. If your weather is cooler around the time that your 2 good beers were made, i'd bet that as a contributing factor
#2 - Stubborn infection in your fermenting/bottling equipment:
Some infections are damn near impossible to kill. Your yeast would outcompete the bugs during regular fermentation, and you might not notice them at bottling. However after priming sugar and a few months in the bottle, they'd have time to grow and cause problems. I had a few of my lower ABV english bitters go south because of this - half the bottles were good, and half had this weird, bitter, twangy aftertaste. I had the same problem with kegged beer until i thoroughly scrubbed everything, soaked in PBW, rinsed, and sanatized. The only way to get rid of infected plastic, however, is to get new plastic components
#3 - lingering speck of something in bottles:
If you're getting the problem from every bottle, this is not the issue. However a little dust is all it takes. This also goes for the jugs of RO or distilled water - you need to iodiophore the outside of the jugs (60 seconds contact) before you open them and pour them in.
#4 - Oxidation due to temp control around bottles
According to JZ, bottlecaps are not a perfect seal. If your bottles are in an area where there are a lot of hot and cold changes (high temp during day, low temp at night) it will change the pressure in the bottle, and that can suck in oxygen from outside air. I doubt this is your issue either, but it could be a contributing factor.
#5 - cleaning residue left in bottles or on equipment:
You should be able to tell this one right away. I think this is a partial problem given the way you explained an off taste from trying a bottle during bottling. Some people hate soap and water ... I've never had a problem ... but you have to rinse the bottles REALLY WELL. Also, residual Idiophore leaves a nasty taste. My recommendation (if you can get it) is to switch from soap/water or one step to Powdered Brewery Wash. Soak your bottles in it for 30 minutes, rinse multiple times, and then sanatize with San Star, Idiophore, or ez clean (last resort). Also, if you can get a sanatizer injector and a bottle tree. The injector will use less sanatizer, and the tree helps it drain out. You want to use no more than 1/2 oz per 5 gallons of water.
#6 - you're getting contaminated tap water or tap water flavors in the beer:
I hate to say this because of $, but if you're not already using the cheap(ish) RO water to mix all your cleaning chemicals, and to rinse your bottles out. Cloudy sanatizers are typically bad sanatizers (at least in the case of san star) - your seeing precipitate from the sanatizer reacting with your water, which means it's less effective.
#7 - Astringency
Technically heat is not the problem here. You could mash up to boiling with steeping grains if you wanted to (not that you should). As a number of folks have pointed out, if heat = astringency, then decoction mash wouldn't be done. *However* heat plus high water PH will definitely cause astringency. If you can't figure out the problem another way, this could be it. Get a test strips and a food grade acid mix. If your water is high PH (over 5) you need to adjust it. One caveat to this is that decoction mashing is done with base malts - this may not hold as true for steeping malts. Just to be safe though, get your temp down to 150ish.
8 - Extract twang
If your LME is not pre-hopped, you probably don't need to add it at 60 minutes. It's already been mashed, turned into wort, and boiled down to syrup. Putting the LME in for less time (last 20 minutes) should be fine. If the LME gets on the bottom of your pot it could scortch, which will leave a bitter, almost burnt flavor.
9 - Boil:
You could be seeing a problem if you're not getting a good boil. Is there any way you can get an outdoor cooker and propane tank out where you are? It'll save you a lot of sanity (much faster heating times), and you can do full boils.
10 - Old Hops
I can't tell you exactly what old hops taste like, but I've read that it ain't good. If your hops aren't coming in a foil reflective bag, or there is an indication they're over a year old and haven't been refrigerated, that could be causing some of your problems too.
11. Put beer pot in ice water, rather than pouring ice water in beer
This is painfully expensive, and less effective, but it will keep any bugs from the ice out of your beer. Chill the boiled wort first, then add your jugs of Distilled water (cool them in the fridge) and you should see a significant difference.
12. Boil with the lid OFF
I doubt this is a problem with extract, but I've heard that you can get "DMS" stuck in the beer if you don't let it boil off.
Sorry I couldn't be of more help - I've traded my beaches for rivers and bridges. Good for brewing, bad for vitamin D production!