...where a seller that uses the NICS is immunized from civil and criminal liability
if a firearm sold legally is later used in a crime.

...for those folks that are suing various gun sellers and makers, I believe there is at least one group, family or individual with actions against a party or parties to a sale where the NICS process was involved and followed and failed to stop the sale. Why is/are the FBI, state police or other involved agencies not named in the suit?

I like the idea of allowing law enforcement access so a seller can go to the local PD or sheriff and have the buyer cleared.

can be subject to felony homicide charges. I'm thinking this would be akin to the charges leveled against a criminal who participates in a robbery where a victim or accomplice is killed even if they did not participate in the actual killing.

...the commission of a violent criminal act should face charges such as accessory, aiding and abetting, conspiracy and up to the same charges as leveled to the perpetrator. The type of charge should depend on their degree of participation in and knowledge of the crime.

As I understand it the problem with most charges like straw purchasing is that the straw dealers involved are offered immunity or greatly reduced charges in return for their help, testimony and/or information leading to the convictions of the other shooter, killers and perpetrators.

Often in cases of murder the only actual witness is the criminal. The conviction then hinges on connecting him to murder weapon. (The weapon itself can be generally be linked to the crime via forensics if it's recovered.) Smarter killers take forensic countermeasures like wiping prints, removing the ammo and discarding the gun. A straw buyer is sometimes the only link.

This sounds like a very good idea but I can also see some possible problems.