Reducing Tree's json editor ability

Hi I am using 3 modes while loading my data into json on my webpage,

I wanted to reduce tree mode's ability....

I actually wanted a form like mode with insert buttons on left.

Basically, I wanted to reduce a user's ability into changing key side of string, and only allow it to either add more key value or edit the existing value side of key.
I tried adding css in form mode, but it seems like it doesnt support it...

Any other way of achieving it? or making changes to tree mode to make key side non-editable?