Applying formula conditionally on a Excel cell

I have an Excel column which is a dropdown. This drop down has IDs which gets populated based on TRUE or FALSE of another column. When TRUE, the IDs are listed here and when FALSE , the list is empty. The IDs in the dropdown are from 2 separate sheets .I have a helper sheet where I have the IDs and related details from both sheets joined together using VSTACK. Now on the main sheet, when I select ID from the dropdown, I need to populate another column ID_NAME from this helper sheet. When the dropdown list is empty, there are manual values in the ID_NAME column which should be left as it is.

I tried to apply formula conditionally like this

=IF(Y3=TRUE, IFERROR(VLOOKUP(T3, Helper_sheet!A:B, 2, FALSE), ""), IFERROR(XLOOKUP(C3, EMS!$C:$C, EMS!$Q:$Q, ""), "") )

where Q is the NAME column which i want to populate, Y3 is TRUE or FALSE, Helper_sheet A is ID and B is NAME,T3 is the dropdown cell and C3 is another Id i am associating with FALSE values which don't have drop down IDs and trying to preserve the NAME for them in Q.But this formula results in wrong calculation and circular reference error.How do i preserve the manual values in the name column which has FALSE mentioned in Y and has no ID on the drop down when the rest of the cells against TRUE should fetch values using a formula. I don't have VBA, tried the python in excel but no results yet.Once the NAME column is populated,I want to keep the NAMES against FALSE editable and TRUE non-editable.Any help is greatly appreciated