My main question right now is about sealing and insulating the sill bays. In videos and demonstrations I see people pressing the foam board all the way back against what I guess is the rim joist. However, my house has a single large beam directly on the brick piers, which supports the joists and studs. No sheathing, just siding on the other side. First picture shows this. 

Because I figured I needed some air behind the siding to avoid condensation there, I’ve been putting my foam board closer to the interior edge of the “sill” like in the second picture. 

But, I’ve found the wall cavities are completely empty, except for a few places where the previous owners had batts put in while doing other repairs. Third pic shows one of the cavities. Do I need to be sealing the bottom of these before sealing each sill bay with the foam board?