Hi Karen
Once you have viewed a search results page or an actual image, this is saved for you to view again at any time. If you look at the very top of the page, you have a number of buttons:
Viewed Images - Previous Searches - Timeline - Shopping Basket - My Details - Logout
If you have viewed an actual image, you will always find it indexed under 'Viewed Images'. Click on that link for the index, then for each entry in this section, just click on 'View' to bring up the image again on your screen. (You might also want to save key images elsewhere on your computer, I think you mention that have already done this?).
As above, to view previous search pages that you have already paid 1 unit to carry out and view, just click on the 'Previous Searches' button.
Regarding Agnes McKerlie/Matthew, I couldn't find a death entry on Scotlands People post 1855. She may not have died in Scotland (they were in England from what you have found in 1851) or she may have died before the start of official registration, pre 1855. I did the search for her as M*c*rl* for the maiden name to try and pick up on variants.
Monica