There is good piece information about this issue here :
https://www.bbc.com/sport/formula1/51739796
One of the things you will find here is the following text about the vortices creaed by the J-vane or cape, as well as the y250 vortex itself:
To create a car that really handles well, consistent positioning of the vortices is vital.
Regardless of whether the car is driving through crosswinds, rolling around corners, or bumping over kerbs, it is important that each vortex stays as close to the optimal position as possible. Otherwise, the amount of downforce, and hence grip, the car has will change from corner to corner, and lap to lap.
This is one thing drivers hate because it affects their confidence and trust in what the car will do, and makes it harder for them to extract the most from the car. It is here that some of the pros and cons of the cape verses the J vane start to emerge.
They introduced the cape for the first time at RB. It looks like this is what Marko meant to say: The did not copy
-the desired effects of- the Mercedes nose well enough.
Changing the nose + cape or cape only will most likely influence the bardgeboard area behind it. But we do not know whether it's only the cape or the bardge board area of both where things go wrong. Worst case scenario is that modifications are needed on both. And that takes time.
I would predict a modified cape and some smaller changes in the bardge board area to start with at Silverstone. Possibly, the changes are into two different directions: one on Max' car, the other on the RB16 of Alex during FP1. From there, they could decide to go for the one or the other.
About the flexing parts: I do not think the flexing itself is a causal factor. It is merely a result of losing underpressure under car at some point and gaining it at another. That causes the flexing back and forth/up and down. If they stabilize the creation of downforce, it will still flex though. But it will flex more continously and as it was meant to.