Forms want one attachment. You have a PDF form and a photo of something. These are different formats, so there is one step in between.
Merging joins PDFs to PDFs. A JPG is not a PDF, so it has to become one first. That is the step people miss, and it is why merge tools appear to reject their photo.
Crop before you convert. A photograph that still includes the table it was resting on becomes a PDF page that mostly shows a table, which looks careless on an application and makes the file larger for no reason.
If the combined file is now too big for the form, compress after merging rather than before. Compressing the pieces separately gives you less control over the final number.
Yes. Turn them all into one PDF together, then merge that with your document.
The order you arrange the files in before merging.
No.