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\ohead[]{\pagemark}\cfoot[\pagemark]{}

Should work

\printbibliography[title=wombat]

Welcome to the forum.

You can click the help button on the Overleaf website. They are replying fast.

What is shown in the figure as equation four can be done using the cases environment of package amsmath.

You can use the geometry package to use any papersize you want.

\setlength{\headheight}{19pt}

I wonder if you could have managed that by simply using google.

Place a percent sign at the very beginning of the inputenc line.

\DeclareLanguageMapping{english}{english-apa}

The unused global option issue is not related to the bibliography. There is a comma missing. It should be onehalfspacing,headsepline. To get the bibliography working, use : \addbibresource{Mendeley.bib} . You should give your bibliography a better name though. Please have a look at https://en.wikibo...

I told you before: you do not have a twosided document but set the options for a twoside document. This is pointless.

I cannot help with the other stuff, you are just providing shots but no real clue what really happens.