The main chain would be more of a backbone chain being used to settle side chains and other major transactions periodically. Bitcoin would be primarily used as a store of value and transfer of major value. This is in line with you thoughts of bitcoin being a reserve currency. We shall see how it plays out.
That can't work.
Mining has to be paid for via usage fees, not the block subsidy. The only way that works is if the cost is amortized over an very high transaction volume.
Again, forcing most transactions off-chain also denies the benefits of Bitcoin to most of the world's population.