How to analyse data (rach to conclusion) Minitab+Statistics Help Needed

Dear Friends,
I have collected data on Box-Behnken design [MATLAB Simulation Data].
Please see attached the excel sheet.

Some questions to me
1) Adjusted R-Sq is less for Y2. Also it is not in agreement with Predicted R-Sq.
1.2) In such case is it necessary to fit cubic model to enhance value of Adjusted R Sq
1.3) If yes, how it can be done using Minitab.
As per my knowledge, Minitab gives model fitting option upto quadratic level with these options DOE>Response Surface Method>Analyze the Results.
How to fit cubic or higher order model using Minitab.
2) With current status, composite desirability is put and conclusion is made.

Analysis for Y1 is understood. For Y2 I am in trouble.