When you were Muslims, whether practising or not, did you ever pray Salat Al Istikhara?
Yes
And if so, did you see any signs following such prayer?
The "sign" that tou are talking about isn't like the naked native american in Waynes World 2. It's usually how you feel about a decision that you need to make. Even without the assumption of istikhara, how many times have you been in a situation where you needed to make a decision but couldn't? You then thought "I'll sleep on it" Then in the morning you are in favour or against the decision? The assumption of prayer or istikhara wasn't present.
I'm wondering if that can be proof of anything? Like proof of the existance of Allah?
I really do think it's a non-sequitor. One must be completely objective and avoid any kind of solipsism when trying to see that as a proof. Look, there are people around the world who are non-muslims but believe in a God from whatever respective religion they belong to. Does the answering of their prayer mean that Yahweh, Ram, Krishna, Ahura Mazda and Zeus exist?
Did you ever feel any of your Du'as was actually answered? That you were ever guided?
Yes, there was a prayer that was answered and it involved the decision of another person. If one thinks further, they will see that the prayer usually involves a decision which is 50-50. It'll happen or it won't. Then one must also see if what you are asking for, is contingent upon something else other than the prayer? Then there is the type of reaction that comes from a prayer not being answered, which is that "Allah knows best, maybe his not granting me this dua this time has some sort of benefit for me?" So, when an answer like that is given, you are not even given a chance to think that you might be praying to a non-existent entity.