Replace all the foam when you replace one strip.
Good foam will spring back to it's original height immediately after being compressed. In a camera this old the mirror foam may not have much life left and when it deteriorates and gets sticky it's a PITA to get residue off the (front surface) mirror.

If your screen is split image, it will have either a very fine horizontal or diagonal line in the center. If you focus on a vertical line and have two images in the center, that's split. Microprism is also very common, this one the center breaks up into small prism like areas. To focus just adjust 'til it's smooth. Frequently you can find a comera that has a split center surrounded by a microprism collar.