For all the info you need on SAK, the app that Siptang
mention (a backup tool that is installed on the linux partition), you can check out THIS
thread. You may not need to sift through that though as the app is made by CJPC and seems pretty sturdy, and also has a decent readme.
I havnt used it personally, but i believe it is the most commonly used, easiest method.
Now for some answers to your questions, keeping in mind that im not exactly experienced with backing up Blu-ray's themselves, but have backed up many isos etc before.
1- I believe that you can get SAK to split the file for you, but i see no reason why you shouldn't make the split sizes say...3.9Gb, as long as you leave a little gap room on the DVD.
2-If the iso is less than a DVD, rar it anyway but dont bother splitting. Ive had instances in the past where an image gets corrupted when being burnt, but a rar will keep it in tact.
3-Winrar or 7zip are fine to use.
4-par files are always a good idea, especially with large files like these. Although because you are burning the files to dvd's they may not be so important. You aren't in much danger of losing a file as you would in say, P2P.
Hope i helped a bit, give a shout if you need anything else, although i think ive reached the span of my knowledge.