6. Can I upload multiple documents for the same category?

 Yes you can. If the document you want to upload is a contractor document go to Documents section and select 'Upload Documents'. If the document is a plant, or employee document go to the entity details and scroll down to the documents table and click the 'Add Documents' button on the top of the table. If the document is an equipment request go to your equipment list and click the upload document button/icon beside to the equipment. In all the above cases select the proper document category, fill in all requested details and repeat the process as many times you have to. 

Another way you can do it is to compress all the documents you want to upload. Add the documents to a .zip file and once the documents are compressed you will upload the file as an ordinary one selecting the proper document category.