Hello,
Thanks for reaching out and for your interest in CryptPad!
As far as I understand, your proposal would only displace the problem of trust from the server, to the application you downloaded. Which could be also altered in many different ways.
However, we understand the necessity of reducing the amount of trust given on the service operators. While we didn't came up with a solution yet, it's something we have in mind and will try to work on.
Have a good day!