I think you are on the right track to reduce the jump to the rifling.
Dangerous game rifles often have humongous throats, to ensure reliable chamber when a buffalo is charging you, or some such thing.
The problem with a 0.400" long front band is that when it is engraved by the rifling, the displaced lead has nowhere to go. This increases bullet distortion and can hurt accuracy. It would be better to have grooves out there. You don't have to fill the exposed grooves with lube.
I assume you will be crimping to prevent bullet jump in the magazine, so if you go with exposed grooves, then you'll have to plan a way to crimp in a lube groove. The planning part is hard because the design program isn't set up to figure on crimping in a lube groove.
Regarding two lube grooves or one, I think lots of little bands and grooves tend to shoot better, however, big bands fill out easier, especially on a heavy bullet.
If your 0.450" bore riding nose contacts the rifling when chambered, that is the main thing for accuracy. You may be able to make the nose 0.001" larger than the bore so they will engrave slightly.