I want to replace power door locks with manual locks, not vice Versa

There is no difference in the manual or electronic systems in the lock cylinders themselves. The difference is that they don't have the lock motors on the latch assemblies. From your description it sounds like you need to lube the latches and linkages to allow the locking levers to move. Then get corroded and don't move very easy. The picture shows the lock and handle linkages. For the outer lock cylinders get a some lock lube, it is powdered graphite that you spray into the cylinder to lube it. Don't use liquid lubes as they draw dust and will bind the locks. For the linkage you have a couple options, the easiest is to put the window down then use a plastic wedge to move the window felt out on the outside of the window. That will usually give you enough room to spray some lube like break free or wd40 in using a straw on the nozzle. Or you can remove the inner door panel to do the same thing. Being an older vehicle you will likely break some of the plastic retainers doing this as they don't age well.

Once lubed use the key and interior lock button to move the lock and work the lube in.