I think you are asking the wrong audience. We can't speak for Oracle. This is the public-facing space for JDev & ADF. All you get here are personal views fo other users.
Here is my personal view or answers to your questions:
1) Yes, 126.96.36.199 is a stable version. A 'smooth' update can't be guaranteed. It depends on the code you have written. This code we don't know (neither does Oracle). You have to migrate to 188.8.131.52 and see what you get. I migrated projects without any problem, but other projects and problems which we needed to address. For the groovy problems, you should file an SR.
2) no comment other than that you should ask this over at https://cloudcustomerconnect.oracle.com/pages/home
3) no. We had this conversation over and over again already.