Nice, how you've chosen to make your runners. Is that ash wood? And is this one of the two runners or do you saw the two from the one once you've got the shape?
I answer your, what I take to be straightforward questions this time in reverse order. First of all with a picture where I have highlighted the obscure handle on the outside of the door.
The parimeter walls of the building are made up of what they call kloostermoppen which refers to a particular soft-fired brick format,(22x10.5x5.5 cm) and lime mortar fired from seashells. Probably the clay of bricks was dug and fired at or near where the church is standing. The tower is, lets say built along side and independent of the original building. The timber framing of the tower begins at the third level, the level in which the bell is contained, and stands on bricked walls more than half a meter in thickness at that point. I'm not sure how it is anchored but the joists there are massive things, around 30x30 cm I think.
Here, because of the earlier influence of the sea, the churches were build on top of man made mounds of clay so the church and the old village stands higher than the surrounding area. This is a cultural/historical characteristic of the landscape in this part of Friesland.
Details for me of the roof construction are a bit vague at this point. I recall only the spaar construction, simple rafters of round and sometimes roughly hewn poles. And it is by no means original as the walls were made higher at some later point. The impressive thing is the upside down boat, the actual covering of the chapel below. In this little film, plucked from the internet, you can see this from the underside.