We do that frequently. Hot spots develop in our oven too, but we control the thing by just swapping the bread from center to edges twice along the process.
I think adding a "thick metal plate" would do too, but the toasting time will increase (or at least the time needed to pre-heat the oven will be greater).
HTH!
The stone will be fine kept in your oven all the time, but there are trade-offs. The biggie is that your oven will preheat slower than you're currently used to--potentially quite a bit if you get a large or heavy stone. And you will really need to make sure that the stone has reached the desired oven temperature (by preheating longer) or the way things cook in your oven will be thrown off. The oven may claim to be fully preheated before the stone is really up to temperature because the thermostat just measures the air temperature, which can be higher than the stone, or even the sides of the oven for a while.
The upside is that your oven will retain heat better and generally be more even in its heat. So if you are willing to properly preheat that stone, you'll probably benefit in the long run.
As you've divined already, you'll have to put it on one of your oven racks as low as it will go toward the bottom of the oven. You don't want to put it right on the coils, but you want it as close as it will go. This means you'll have one less rack available to you. In practice this may not matter much, but it's a thing.
I wouldn't recommend putting a pan directly on the stone, but that's because it would completely change how heat gets to the pan. When a pan is on a rack, there's heated air circulating around from all sides. If it sits on a preheated stone the pan now has direct heat conducted to the bottom--which will change how things cook, and many things you'd cook in the oven don't want that direct heat.
Broil mode should be fine with the stone in the oven. I wouldn't want to put a cold stone right up by the broiler as the thermal shock might crack it, but with it at the bottom of the oven there should be no issues, even if the stone isn't heated up.
Best Answer
Flames should not come out of the vents inside the oven. This should not be happening. It could indicate an incorrect or faulty regulator, or ventilation problems. In addition to fire, you may also be at risk of carbon monoxide poisoning.
Your oven is not safe to use. Don't use it until you can get it fixed. Since you just bought it, contact the manufacturer/reseller/installer. Hopefully those are all the same entity, in which case there will be no way they can shift the blame onto each other. Expect whoever made the mistake to leap at the opportunity fix the problem, and feel extremely lucky that they aren't being sued by your next-of-kin. (That's not to say that they actually will do that; just that you should expect it of them, and communicate that you expect it of them.) If you had it installed by an unqualified handyman, find someone more qualified to inspect and repair it.
If you installed this yourself, and/or are planning to fix it yourself, DIY Stackexchange would be a good place to look for advice.
This issue can happen if a natural gas stove is converted for use with propane, the oven wasn't adjusted to compensate for the change.
Note: AMtwo, Sneftel, mroll and Joe pretty much answered the question in the comments section, so I just summarized their comments here.