They definitely aren't stupid questions Teresa, but they are really really hard to answer. Here's my best guess!
In the beginning, most RRMSers find that their symptoms improve very well during remission, however, as time goes on, the body gets worse at repairing the damage and the symptoms don't improve so well and start to accumulate. So, some years on, most RRMSers have many long-term symptoms on a daily basis (just like progressive MSers). Then they'll have a new relapse, which may or may not improve after some length of time. Etc. If the RRMSer starts on a standard injectable DMD within a couple of years of diagnosis, it seems to delay the onset of disability - when the symptoms start accumulating. The effect isn't big, and it only works if the DMDs are started early. So, basically, someone who starts early will become disabled later than someone who doesn't (on average).
The effect of DMDs varies a lot though. Some people do really well on them; some don't. Some people develop "neutralising antibodies" (NABs) to them so go from doing really well on them to them having no effect. The average is a 30% reduction in the number of relapses. The remaining relapses are less severe. Relapses can do significant, permanent damage, so avoiding and dampening relapses is good news. But relapses do seem to be a separate process to progression. Standard DMDs don't do a whole lot of good for progression. However, the stronger DMDs do help and there are trials going on at the moment with Gilenya and Tysabri and progressive MSers and I think they are having an effect, so perhaps the stronger DMDs do something for progressive MSers too.
One thing that DMDs definitely do is help to prevent MS fully developing: DMDs are now prescribed to people with CIS in several countries because they significantly reduce the number of people who go on to develop MS. Rebif has just been approved in Europe for CIS. Whether or not NICE approves it we'll have to wait and see, but I doubt they will - they don't even like prescribing it for people with definite MS.
RRMS vs SPMS vs PPMS and quality of life? If I had to pick a "type" to have, I'd go for RRMS, but I know people with RRMS who are very badly disabled and people with PPMS who are not, despite having MS for similar lengths of time. People with SPMS who have no pain or fatigue, but can't walk. People with RRMS who are bedridden, but others who run marathons. There are just no rules with MS.
If I was PPMS, I would be taking LDN. Have you looked into it? See the ldnresearchtrust website. There is no DMD for PPMS at the moment and there are several PPMSers on here who swear by LDN. There are also those who don't, but nothing ventured, nothing gained.... You could also look into getting onto a clinical trial. There are more and more being run for PPMSers. See the Multiple Sclerosis Research Blogspot.