You are asking questions about the word 'exist' which presuppose it has a single clear-cut meaning- it doesn't. The word can be applied in different ways, and you will get into all kinds of muddles if you confuse them.
Objects that physically exist, in the way that wind, apples, electricity, fork-lift trucks etc exist, can have physical effects. An imaginary fork-lift truck cannot lift a real barrel of molasses. So yes, to exist in a physical sense means to have the potential to cause physical effects.
Imagined objects, or ideas generally, can have physical effects. Consider the impact that the idea of god has had on humanity, for example. However, you need to unpack that carefully. You might consider, for example, that the idea of god is just something we associate with a particular configuration state of the particles that comprise our brains, ie something that physically exists, so the physical effects that the idea of god has on people has, in that sense, a physical cause. You might take the same view of other non-material causes of physical effects in humans, such as doubt, optimism, an imagined ghost, etc etc.