I think there is an extent to which it feels, not impossible, but very difficult to prepare for the step-up to Test cricket. India's pace attack, when firing, offers absolutely no respite for the batsmen and it is just not realistic to expect a county team to have such a quality sustained assault. Same for pretty much all countries atm - where do you rest against Boult, Jamieson, Southee and Wagner - or against Cummins, Hazlewood, Lyon and Starc (where the recent weakest link bowls at 150kph) or even full-fit England who could boast three or four of Anderson, Broad, Jofra, Robinson, Wood. I think even A Team attacks would feel a huge step down from this, hence the huge amount of runs that Shaw and Agarwal were able to pile up, in their run up to India selection.
I think this is where people who are very successful domestically do struggle. It may not be that domestic attacks are poor (they aren't) or pitches are easy (as discussed, often not) - just that there is a much higher quality of bowling in Test cricket. What was interesting to see is how Rohit got so many runs off Sam Curran in the first innings whereas he was going fairly slowly until then - because there just wasn't anything to score off. KL Rahul was striking at around 20 and no one can accuse him, or that innings, of being particularly negative.
Re: techniques - I think it is a good point that it really feels that the likes of Nasser and Athers did agonise over the "correct" techniques and would try to adapt these to limited overs. These days, it feels the limited overs techniques are being applied to the Test format. Sir Alistair Cook once spoke in an interview about how Test batting was straightforward for him because he had two attacking shots only, the flick and the cut. Potentially, on occasion you would see the defensive forward push down the ground. This made decision-making for him far more straight-forward.
This being said - I think England could well come back in this series as India fatigue and I think there is a global decline in Test batting around the world.