1/ Ok one simple functionnality that could make life easier when combining several pdf, in windows explorer, select all the files you want to combine, right click and combine in PDF Architect 6.
2/ How to add a simple Date field (today date for example) in a form and duplicate it over all the page ?
3/ How do you edit an already existing header/footer ?
1/ Thanks for your suggestion, I will forward it to the colleagues working on PDF Architect. In the meantime, you could use PDFCreator for this (Select files -> right click -> convert with PDFCreator -> Merge -> Merge All). Will take still more than one click if you're not using an automatic default profile. 
2/ & 3/ Please see if you can find answers in the PDF Architect user guide or contact our PDF Architect Support team for further assistance: https://support.pdfarchitect.org/customer/portal/emails/new