The edge connector plug was also given a clean.

This connector requires some care because it can be damaged with small force. Cleaning is more cumbersome but focusing on the contact points to the cartridge were done. This is because the edge connector plugs onto the motherboard with a solid amount of force contact. Compared this with the cartridge which sits on the connector and only has a relatively smaller force applied to the contacts by the cartridge locking mechanism. This type of connector applies almost no sliding force onto the edge connector which can help self clean the contacts by the wiping action. But on the NES the cartridge simply sits on the connector and lightly pressed down. Now this is good for longevity of the contacts because the parent material is not slowly scrubbed away with each insert and removal action. But on the flip side, there is virtually no self cleaning. This should be a primary focus of your problems before venturing into deeper repairs.

The unit was further cleaned with a fine brush and the chassis with all items removed then washed in warm water and dried.

Dust on the motherboard removed with a brush. If you require a deep clean then isopropyl alcohol may be used.