The top contact on the batteriser is made from sprung loaded fingers (say 0.05mm thick?); board 0.3 or less and 0.1mm for the neg term sleeve. So their total additional length will be of the order 0.4 max.
As you mentioned, you are going to have a weak board after isolation milling, but perhaps potting the component side after assembly would make it more robust? In batterisers deign I guess the sleeve adds a little rigidity to the board also.
I think the spring fingers is something that got right. They will extend when you need them, at the positive end, and compress when they are not needed, for example between two cells. It would be hard for me to make something similar
I am not sure how they would mate with the spring contacts in the fluke 337 clamp meter.
I think that potting the electronics is a good idea. It is not something that I can do.
I am just doing this my (our) entertainment. It is the smallest board that I have ever worked on. There are some challenges making boards for 0.45mm pitch DFNs and soldering them.
By doing it, I am exploring some of difficulties that the Batteroo team might have.
I think that it is feasible to build one for an AA battery that can deliver about 500mA.
The experiments that I documented in this post:
https://www.eevblog.com/forum/blog/eevblog-751-how-to-debunk-a-product-(the-batteriser)/msg809012/#msg809012is the most significant result. It shows the maximum power that can be extracted from AA cell as it is discharged. This is governed by the maximum power theorem and the ESR of the cell. The inductor will add about 100m Ohm to the cell's ESR. This shows that it can not work at the high currents that they are promising.
It is going to be interesting how this plays out.
Regards,
Jay_Diddy_B