No offense taken, I would think - these are very good questions.
In general, folks with bad knees should avoid shikko and suwariwaza to prevent additional wear and tear on the cartilage.
Folks with mobility, range of motion or pain issues need to be allowed to take their time in learning to be comfortable getting down/getting up, being on the ground, and learning to roll and fall.
I lead a separate class for people with those problems, but I don't recommend doing so unless the person leading the class know how to do so. There are lots of easy ways to adapt "regular" aikido to no-falls aikido or slow aikido. In fact, there is a thread on aikiweb here about a seminar in Virginia at the end of this month on that subject. HERE is link
I would be happy to communicate more with you either here on this thread or via email. Actually, let't keep it here on this thread you started, since others may also continue to contribute valuable info.
Oh Jun keeps reminding me I have a column on the subject :-) http://www.aikiweb.com/forums/showthread.php?t=22821