You are correct on your understanding of the 777 pellets. They are formulated to shoot like regular Pyro pellets, so they ARE weaker than an equivalent measure of loose 777. If I had to give a "seat of the pants" estimate of an equivalent charge - I'd say the 3 777 pellets are probably about the same as 120gr of loose 777.....................and really, 120gr of loose 777 is probably a max load anyway - going above that is probably not gonna help your shooting any and COULD be dangerous over time IMHO.

I really haven't tried my luck with 777's corrosiveness - but seeing how easy it cleans up, I would have to guess that it is probably less corrosive than Pyro and MUCH less so than Black. But regardless of powder, Muzzleloaders need to be cleaned aggressively or there will be problems - they can't be treated like smokeless firearms..................not if you want to keep them serviceable for years to come.