ericnzet: not sure about the updated background/declaration form. In fact our officer didn't even look at it, but not sure about electronic version.
canada74: you should have submitted a similar letter from your employer at the time they requested documents, in my opinion that (the original if you still have it) should be enough if you find it hard to obtain a more recent one.
What we did was just get a new letter confirming we were employed (just 3 lines really, only name and dates, no details at all) from employer, but still brought the other more detailed letter we'd submitted with docs, and explained.
Bottom line is, if you have "old" letters of employment with details of duties etc do bring them, and be prepared to explain that you could not get a new one. If you can get something very basic that's recent, the two together should be good.
Again, this should not be a selection interview, just to check if the docs you submitted initially were genuine, so they should be happy with originals of what you submitted before. Try to bring extra documents to make up for the ones you can't show.
In our experience the officer does have a checklist but it's more of different sections than a list of items, for instance s/he could accept other documents for proof of employment and tick the box even if you did not bring the exact letter they asked for.
As a case in point, my wife (main applicant) had been unemployed for 6 months at the time of interview, so no recent payslips etc. So instead she took payslips and bank statements for the past year (instead of just 3 or 6 months as asked) to show them she was clear. We didn't try to cover up or hide, we just said it when he asked about employment. Be honest, that's the most important thing. If you have anything that's different from what they ask, make sure you have other documents to make up for it. Maybe you spouse can get a recent letter more easily, in that case even if the officer didn't ask for it bring it along and it may make up for yours. The officer should be flexible.
That's my advice anyway.