For what it's worth, I have extremely hard water with a high pH and I have always been able to keep soft water, acidic fish provided I acclimate them properly.
What happens with fish kept in too high of a pH or hardness usually is not premature death (unless they are a really sensitive fish, or perhaps a coral or something). Usually they just won't breed; potentially, they won't live quite as long (kidney damage has been observed in neon tetras kept in hard water; though those specimens lived a typically long lifespan), but dying after a couple of weeks does not sound like fish being in too hard of water. That sounds like disease, poor water conditions of some other variety, or even the issue of the water just being too hot. Potentially, it could be an issue of shock.
You said the LFS is 3 hours away. It's possible, very possible, that their water conditions are very different than yours. Drip acclimation, potentially even several hours worth, may be necessary to prevent shock. This sort of shock doesn't always kill instantly; sometimes it just makes the fish that much more susceptible to disease or causes them stress, permanent damage, and death days or weeks later.
ESPECIALLY when water conditions are different, acclimation doesn't mean simply floating the bag. Mixing water is essential.
I have had Lake Malawi fish, which is the 'perfect fit' for my super hard, 8.4+ pH water. But right now, everything I've got is a soft/acidic water fish. And I have not experienced these issues. One thing is for sure though, drastically lowering the pH quickly will just create more shock.
The best way I've found to naturally soften water has just been plenty of good driftwood. I like it because it only makes a small difference, and it seems to do it slowly. I'm a firm believer that consistency is an order of magnitude more important than having similar water parameters to the fishes "homeland". Big swings from trying to change those parameters will kill fish fast.
FWIW there's a lot of things I'm willing to cheap out on. Lights on non-planted tanks, power heads, stands. But heaters, no way. My preferred heating setup is two heaters, each about half the wattage you need. Two, together, get the temperature up to the appropriate level. If one fails in the 'on' position, the other will shut off and the tank will not overheat, because your two heaters are not, individually, powerful enough to overheat it. (for example, if you determined you needed 200 watts of heat; use two 100 watt heaters instead of a single 200 watt heater). I have two heaters in all of my tanks except for my 10 gallon, where it's just impractical. And all of my heaters are good quality heaters. Cheap heaters wreak all kinds of havoc. There was a fiasco some time ago of a particular brand getting water in them, which turned to steam and built up pressure, and exploded! Eek! I like Eheim heaters but, any good quality heater will do.