This might not be the right board for this...
Suppose I place a sell order for 1 btc at $45k on your favourite exchange. Next, I place a buy order for 2 btc at 46k, will the exchange execute the trade?
This scenario feels pretty weird to me, I don't think it makes sense for the exchange to forbid the sell order from being submitted, but I also don't think it makes sense for it to execute the trade. What is the expected behaviour here?
That's weird because you're making a scenario that's obviously going to be filled. Since your buy order is @ $46k then that will be filled with the sell order of 1 btc @ $45k. You hit it at an exact or lower price. But there will be a remaining 1 btc buy order @ $46k since the sell order is only 1 btc not unless there will be another sell order that shall fill with the same price. The trade will be executed automatically since you put that in order and that's going to happen no matter how you won't believe on it because that's how trading works.