Good evening sorry for the delay in responding.
There isn't a standard facility to export\backup files via USB or Network to a PC, in order to meet the copy right protection requirements. The reference I believe you have seen regarding a backup, suggests the content is backed up to disk ("As a basic rule, save important content to a disc as backup.").
I am aware of some reports where consumers have been able to do this, such the by follow the steps listed here, but this isn't a supported processes (and therefore may not work).
With regard to lens cleaning, this shouldn't need to be done very often I often wait until the machine stops playing and starts jumping when playing \ writing disks, I would always use a blueray lens cleaning kit (if any), have a look at the video here which claims why, you can pick up Clean Doctor on Amazon for under £15 here.
With regard to your other 2 questions about a potential replacement \ upgrade PVR, you are correct, you would need to use an analogue input such as scart to prevent copyright protection issues, however this will become more and more of an obstacle to overcome as analogue inputs\outputs are faded out (as can be seen on the latest Sky box)
The 800 series introduces increased storage, and was first range to include support to RGB input, and come with integrated WLAN, and multiple HDMI. In essence the 740 does have all the key features the 800 series offered, so would seem to be the logical upgrade for you (as it also continues to have a Analogue input via Scart).
Sorry for the delay in responding, the question was marked answered so didnt show your follow up question, rest assured there is not additional charge for seeking the clarification requested.
Unfortunately there is not mechanism for backing up and restoring CD titles, other than the approach to export the contents on to disk which as you have already advised is orientated around video recordings. I appreciate this must be frustrating, but I would suggest you look towards using a media centre to mange your CD collection going forward.
The software on the DMR-BWT-700 does not support USB or Network export, so I would advise buying an adaptor as this is not a support mechanism, and will probably cause more issues re-importing the content back on to your replacement.
You could approach a service centre to copy the content, as they would probably be able to remove the two hard disks and transfer the files and config over from one disk to the other, however this isnt a recognised process but at least paying for it to be done professionally you know its not going to invalidate your warranty.
In summary should your HDD fail, you will lose all the data \ information stored on your PVR, a media centre would have RAID which is redundant HDD and reduce the likely loss of data. I would urge against buying a LAN adaptor since there is no supported export facility, but would suggest you contact your local service centre to ask for a quote copying content from a 700 model to a 740 model for example.