Can you have a hypermobile ACL which causes instability, collapses of the joint? And would an acl reconstruction be helpful

Unlikely. It is fairly unlikely to have an ACL insufficiency with an intact ACL ligament. Most people who would have that would show signs of generalized severe ligamentous laxity and also have problems with there shoulders, elbows, etc. But yes, theoretically if you had an intact ACL that was not doing a sufficient job you could do a reconstruction with something stronger.