Charging a whipper two or three times is definitely not going to make it explode; for certain lower-density preparations you're supposed to charge it twice, but even if that weren't the case, consider that one of the chargers holds only a fraction of the volume of the whipper itself (8 g, to be exact), so if the chargers don't undergo explosive decompression then it's definitely not going to happen to the whipper.
As for it possibly turning into butter, that's a more realistic possibly depending on what you're putting inside it. I can't verify whether or not it happens with cream since I've never tried it and probably never will; I wouldn't recommend it, since whipped cream gets very dense to begin with.
But honestly I think your biggest problem is going to be that whipped cream dispensers aren't designed to dispense upward. The instructions are very explicit that you have to hold it upside down in order to dispense; if you hold it right side up then it's just going to vent the gas and make it impossible to get the cream out with opening it up.
Simply stated, the cream is much heavier than the gas, and I believe that whippers don't actually achieve saturated vapour pressure inside. That's why the gas will always eventually end up on top no matter how you orient the whipper; the key to getting the cream out is forcing the gas back down through the cream by depressurizing it, which is why you need to hold it upside down.
So unless you want to eject it from the bottom of the cake, I'm sorry to say that I don't think this is going to work for you.
One alternative you might be able to try is to pre-whip the cream with a stabilizer or make a fondant, then put it into a hollow tube and eject the gas from the whipper into that tube. That way you can guarantee that the "liquid" is actually in the path of the gas, and the gas should propel it upward. I emphasize should; it's going to depend on the density of what you're trying to propel and how good your seal is, and you'll probably have to experiment a little.
The cream whipper relies on gas expansion to work.
When you make whipped cream by beating, you beat fine air bubbles into the cream. The cream traps air and becomes essentially a matrix that holds those bubbles--a foam.
Your gas-charged whipper does the same thing in a totally different way.
When you charge the whipper with gas, there's high gas pressure inside with the cream. The cream will actually absorb the nitrous oxide you put in. Because of the pressure, the gas absorbed can be thought of as really really really small bubbles within the cream. So you have a matrix of gas and cream, but because the bubbles are so small, it's essentially just cream.
Chilled liquids more easily absorb gases at high pressure, which is why it's good to use cold cream and keep the whole unit in the fridge. A limited amount of agitation (shaking) exposes more cream to the gas, improving absorption.
When you release the cream from the device, the absorbed gas expands rapidly. The bubbles get bigger, and your cream to bubble ratio becomes more like the foam that we know as whipped cream. It's really exactly the same thing, only with nitrous oxide instead of plain ol' boring air inside the bubbles.
Why nitrous oxide? As I understand it, it's because it's the cheapest non-toxic, odorless and tasteless gas you can get. Carbon dioxide would almost be a good choice, but unfortunately it's bitter. Not a good match for cream.
Finally, why is shaking too much a bad thing? That one I don't know for sure, but I know what happens when you over-whip cream with the mixer. You make butter. Perhaps the gas or high pressure encourages this conversion, or maybe you're just churning it that much when you over-shake. Either way, I'm sure you've essentially made butter when you shook it too much.
Best Answer
Bob, welcome to Seasoned Advice. Are you thoroughly rinsing whatever chemicals you use to clean it? Also, are you adding any other ingredients, such as sweeteners?
Obviously it wouldn't be the same bad batch of cream over a full year (although that would certainly explain a gross taste...), but you might check the freshness and flavor of any other ingredients.