The MF4580 does have a duplex document feeder. I can see via the specs that the MF4570 only has a single sided document feeder. There is probably a few other items that the 80 has over the 70 but I don't believe the operation of the MF Toolbox will differ that much between both models, and it is this software you have based your opinion on.
Looking at your procedure I can suggest a couple of things to make things less "kludged".
At step 4 you selected Custom 1 and changed several settings for that preset. But then at step 11 you selected one of the other presets - not Custom 1 (you would have needed to scroll down the MF4570 LCD view to see this custom preset).
So what you should do at step 4 is modify the Color Scan and B&W Scan presets to what you want. That is, what file format and resolution the image will be scanned at via the Scan tab; where the document is to be saved to and in what format via the Save tab; and what application opens after the scan has been completed via the External Program tab.
With these settings now saved for the selected preset you only need to select that preset on the MF4570 Scan menu. There is no need to use MF Toolbox again. So, on the MF4570, select Scan > Computer > Color or B&W Scan > OK. This will scan the document to the Mac and save it to the folder you chose previously via MF Toolbox and open it with the application you selected, also via MF Toolbox.
The thing to note is that this operation is for document scanning. If you only want to scan a portion of an image on a page then you should place it on the platen and via the MF Toolbox menu, press the Save button. With the Scan menu displayed, change to the setting "Display the Scanner Driver". This will make the MF ScanGear window appear that looks similar to that used by the MP830 you mentioned previously. And this will allow you to prescan the document on the platen and crop the portion of the document you want.
All that I have mentioned above is shown in the HTML guide included with the ScanGear download. I cannot stress enough to all reading this to take some time and read the manual. It will save you time later when something does not work the way you think it should.