It will be interesting to if Mercedes does indeed do this.
The way I see it, the problem with stalling the diffuser is the lack of downforce causing instability under initial braking, as well as instability ( especially center of pressure) when the downforce suddenly comes back when the rear rises as the speed drops through the stop.
The active Williams of 1992 & 1993 could do the same, but you were allowed to actively control when it happened. Even then, both Nigel Mansell & Adrian Newey both said that the car did suffer from momentary diffuser stall in corners, and the car would step out on occasion. Nigel trusted that the car would still corner, where as Ricardo Patrese was less accommodating to it, and more cautious, which is where he sometimes lost out in terms of lap time.....
In my opinion, the sidepods will be as far back as they can be, but to still be able to pass side impact tests. This would be to allow for maximum area for the air coming off the wider tyres, an easier time to be diverted around them, with the bargeboards. You can't really push all the radiators and coolers forward anyway, as the cars have a strictly controlled front/rear weight distribution.....
I think that the sidepod undercuts will stay, as they help to get the mass of air around the sides easier, and feed into the coke bottle area. I doubt that the flow will detach over such a large radius that the pods have back there. In most cases, the flow back there would more than likely be turbulent anyway.