How to open a hot furnace without damaging it?
There are ovens made to be opened when hot and there are ovens not made to be opened when hot.
From what you mention what the manual says, yours can not be opened when hot.
As there are different parts that go wrong at strong heat loss or getting the full blast of the oven heat, it is impossible to predict what your oven can handle and whether you can adjust it.
Your best bet is to replace this oven with one made to be opened when hot. Your second best bet is to ask the maker or a community of oven users where someone may have more knowledge.