What is tread depth of the tires? 5mm is about 6/32". If your rears are 8/32" and your fronts are 5mm less, you may need new front tires. If your rears are 15/32" and your fronts are 5mm less, then you may be able to just swap fronts and rears and keep on truckin'.
No idea. How old are the tires? How is the wear pattern on the tires?
I usually replace tires when they are around 3-4/32 tread remaining. If there are still several months of nice/summer weather, I may let them run until the end of summer and replace in the fall. Ultimately, it is up to you when you want to replace tires. If they have unusual/abnormal wear, they may need to replaced sooner and you may need an alignment. I've heard the best investment is a $160 lifetime alignment from firestone car care centers.
