I am a stylist at Toni and Guy and have been in the industry 10 years.... but this is just my opinion... here goes.
Firstly the way the second stylist carried out the procedure was kind of correct. Both the colour and the highlight would be applied at the same sitting and then developed and rinsed off at the same time. This makes its easier for the colour placement and also the logistics of running a busy salon. There would be no real benefit of applying the roots first then shampooing the colour off and then applying the highlights. I can't think of a real life situation where this would be best,
Secondly depending on which products were used the colour was probably left on the hair for the correct amount of time! for example: The development time of L'oreal Majiblond would be 50 minutes, L'oreal Majirel 35 minutes etc. If pre-lightener was used it could be left on for 50 minutes! I also think that you last hairdresser was maybe not upto date on her theory... If she thought that the colour could go too dark if left for more then 30 minutes she was probably selecting the wrong colour in the first place. ALL tints (not pre-lightener/bleach) are designed to lift or deposit to a target colour and if rinsed off too early the correct amount of tone may not have been deposited in the hair. If the product has a development time of 40 minutes then it needs to stay on the hair for the full 40 minutes... not just when someone 'thinks' it will be dark enough... like i said if she thought there was a chance of the colour going too dark then she had chosen the wrong colour in the first place. Really thats the fundamentals !
I think your new stylist was doing a decent job and just probably couldn't be bothered to explain all this... which is a shame as it gives stylists a bad name and clients leave the salon feeling deflated!
But honestly I think that you are in better hands now.
Hope this helps.