I apologize not my specialty

It's more depends on what you are claiming in your publication. If you only state that protein of interest present in this type of cell or tissue that you have lysed and its expression is different in different conditions or the protein is well characterized and known to be the marker of a certain process or cell type, then WB is sufficient and it is unlikely that reviewers will require IHC/IHF, however, if the scientific question involves localization of the protein within the cell or tissue than IHC should be performed. In your case, you don't even need to quantify the WB just put the representative blot with the difference and support it with RT-PCR that you already have. I have many of my publications protein of interest that were referred by WB without additional IHC (in some cases WB alone was sufficient). Actually WB considered a more robust method, so if you only present IHC than reviewers frequently request WB or RT-PCR confirmation, however, it is rare another way around.

Might be enough if protein localization is not your objective, However, functional assays would be needed to show activity of your expressed protein.

Yes. If you’re not interested in protein localization or which specific cell type/s express/es that protein.

Or I think it’s okay if your tissue samples do not have high expression of that specific protein hence, you have to concentrated the protein to become apparent or observable (through western blot).