I think you have got this mostly correct, the only problem that I can see is that you are binding the column in your relating table to the values from your SelectPow2Powfee recordset. Since this recordset is filtered to the first item id from your other recordset you will only insert the values for this first item id.
What I did to update it was to just use the bindings for these columns from your select all recordset. When I gave it a try like this I was able to insert a new record into the pol table and every value that was checked was inserted as a new record in the politem table. I'm including a screen shot of the changes I made for reference.
Since you are not using the bindings from the second recordset for the MRT it does not seem necessary to have this recordset on the page.