1) Shimano will keep a cable option. If nothing else, cable operated XTR serves for trickle down technology to XT/SLX/Deore in the future. They still persist with mechanical Dura Ace even though most roadies have bought into Di2 and will buy Ultegra Di2 at the same price point.
2) Agreed. Shimano need to bring back some consistency and reliability to their range of brakes. A bit more modulation wouldn’t go amiss either.
3) Shimano will stay away from 12spd indefinitely. They don’t like playing SRAM at their own game, and Shimano will likely persist with their (to be fair, highly refined) front derailleur options for as long as possible. Worldwide, the U.K. and the US markets are still considered quite unique in their love of 1x drivetrains, and Shimano more than any other company in the bike trade, like to play it safe.
4) The biggest reason for that was to allow 1x setups to use smaller than 30T chainrings. But most people now don’t go below 30T as the cassettes have got bigger! With a 4 bolt setup you can change the chainring in the field, with just a 5mm Allen key, without removing the crank. I’m not sure Shimano are fussed either way, though it does seem mildly odd they’ve doggedly stuck to splined interfaces for brake rotors, but not considered it for spider/chainring mount (not since XTR was still 8spd). Time will tell I suppose, but the redesign of the 4bolt chainring pattern was I imagine all that Shimano are going to offer here for some time.
As steve_b77 says crank based powermeters will become an option soon enough too.