I actually do understand why a new rider might have trouble reaching for bar end shifters or the water bottle. I know I did when I started riding again, but that's why I tend to think that Catrin's issues may have less to do with the bike set up and more with just being new to riding. Without at least seeing her on the bike, it's just so hard to tell. I'd suggest, too, that if the bike really does have squirrelly handling because of the way it's set up and/or in how Catrin's weight is distributed over it, then she would likely sense it all the time, not just when she takes one hand off the bars.
Catrin, I know Jonathan has told you that he would prefer to do a fitting once you have more miles on the bike, but you might explain your situation to him and ask if he might have some spare time to head out to the parking lot and watch you on the bike. If nothing immediately jumps out at him in terms of how you look and how the bike is set up, then I'd suggest that it's really just a matter of practicing.
If nothing else, let's try to find a time where we can meet for a ride. Bring both bikes so that I can at least watch you ride it. If we met at my house, I could even set it up on my trainer and look at your positioning. I've spent so much time with my own fittings that I feel like I have some idea what to look for.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher