Dependant visa extension - SKill Worker route

I am in the same process, I received my ILR on Aug 2024, my dependents will be eligible for ILR on Jan 2026 but their current dependent visa is expiring in Aug 2025. So, now I tried to apply for dependent extension and I have given the following things, @ADMINS kindly answer my questions because I need to proceed with application.

1. UAN number I have given my last skilled worker visa UAN number and not ILR application UAN number is this correct?
2. I know we need to pay 3 years IHS fees although they need only 1 year for ILR, in order to do this in my COS work start date I have given the actual COS work start date Aug 2022 and COS work end date as Mar 2028 ( actual date is Aug 2025) , so it will calculate for 3 years IHS. Is my assumption correct?
3. And it gave three years IHS fees and I paid. But now in next page it asks me to pay application fees as £1636 which is extension for 3 years and above, but I want to pay only less than three years which will be £827.
I have not paid application fees because I m doubtful if my application is wrong, kindly advise me how to proceed and will they refund any amount if there is any mistake in my above assumptions and if i correct them submitting a new application?