Also bear in mind that a shallow concrete trench could be susceptible to heave if laid on clay. It doesn't have the same overall mass as the larger slab so can't resist the movement as much. It'll probably be ok though.
Spacing of the lintels will depend on the span of the timbers you presumably lay perpendicular over the top. If you know the dimensions you can look them up on span tables.
https://www.timberbeamcalculator.co.uk/en-gb/span-table/floor-joists?load=1.5&class=C24
I always thought the risk of non-genuine batteries was fairly low so long as they weren't left plugged in or connected to the tool. May be wrong though.
Impressive work. I've never really considered that woodwork can be a historical re-enactment. Trying to reverse engineer the old techniques is an interesting concept.
Good idea. Might need some clever ideas about naming the items and where to post them. Posting to general marketplace they could get a bit lost, but equally posting to a craft based group would be a slightly closed audience.
Could you not just butt your block work up to either side of the door? You could lap your DPC down and under the frame if you were worried about the detailing.