Sorry, but I don't get it.
Filters are populated immediately after selecting a CI. If you want to show only open incidents of the selected CI, for performance improvement, than you should add the corresponding criteria to the OOTB filter.
Ok scrolling down the form is not very usefull, but what about if you create an extra creation window for your incident that only include the neccessary fields with the CI very close to the filters, then nobody needs to scroll that much.
I will add some screenshots here tomorrow to make that a lille more clear.
With your solution you are forcing your analysts to do an additional mouseclick, and from that, what I know from our customers, this is normally something they don't want to do.
One more question: Are you using console or webaccess?
Why your solution nevertheless is not working, I can't tell.
Cheers
Fabian