There is only 1 solid solution to keep having the problems you described.
Do the followings:
- Purchase the kit that you want
- Once received, open and check the parts over to make sure no damage to the parts, broken parts, or loose parts.
- Close the box back on and store it away to become permanent backlog. You can skip step 2 if you do not wish to check them.
But seriously though, what you describe happens to model kits all the time, not just MGs. That is pretty much why they sell glue and epoxy putty and all kinds of stuffs. Broke one of the part on the big backpack sword of the MG Red Frame Kai few weeks ago because the part became too brittle, was not even trying to much to it. So these things happens all the time, more so if you have them displayed.
There are few things to keep in mind about the kits, personally observed anyway, not based on any actual data.
The plastic from the official kits, while most of time in good quality, do suffer from batch issue from time to time. Meaning MG Epyon from last month might have or might not have the same quality of plastic from the ones months before. That is the problem with mass production. Even if they do have quality control, sometimes those things happened. Similar thing happened to other manufacturers, such as Tomy Takara. There were issues of their top of line Transformers Masterpiece line from before and there were tons of discussion about it.
Another thing would be whether or not you have the kit displayed or not and the locations. If you have the kit displayed, several factors can affect the kit such as temperature, lighting, and whether or not sunlight will be shinning on it directly or indirectly. If you have the kit displayed in a place that temperature differences were big, sudden hot and cold such as window sill for example, the quality of the plastic will definitely be affected. Too hot and plastic an melt and too cold plastic can easily became brittle. For the lighting, halogen or incandescent light definitely will be too hot for the plastic. Fluorescent or compact fluorescent seemed to be okay, but they do emit some level of UV radiation. LED is probably what most people use these days, but to get a very high level amount of light from LED, the more power will needed to be drive to the LED or its driver, and that do still require a fair amount of cooling.
Finally, if you have your kit painted, that does provide a certain level of protection from the environment. But it also depends on the type of paint you use and whether or not you prime the kit before painting. If you use acrylic paint, you probably can get away without priming the kit and the paint should not affect the plastic too much. However, if you use enamel paint, it might be best to prime the kit because enamel paint definitely can affect the plastic if you paint it straight onto the kit.
After all the writing, not sure where this is going anymore… But if you are worry about parts breaking, maybe you might want to invest into the metal build figures. Have yet to own one, cannot say with certainty, but as far as known, their frames were metal or at least similar to that of Soul of Chogokin, where armors might be plastic but important parts were diecast.