Firstly, I have an incredibly rudimentary understanding of how neurons interact etc.

My understanding of memory representation is that it's a group of interconnected neurons that fire in some particular way when the memory is recalled. Assuming this to be the case, what exactly causes a memory to weaken? If it's a weakening of the connections between neurons, say due to a lessening of the number of neurotransmitter receptors (no point replenishing receptors if they're not being used), does the memory seem weaker because the signal literally takes longer to traverse all connections in said group?

For memory loss, do some neurons simply have all their receptors for neurotransmitter X decay and consequently, break the memory? Following this, can neurons have neurotansmitter receptors for multiple neurotransmitters, and can a neuron release multiple neurotransmitters? This would imply that neuron A could be used in the storage of multiple memories. So perhaps a different memory becomes reinforced (more neurotransmitter receptors for neurotransmitter Y), "kicking out" neurotransmitter X?