Interesting thanks for the picture… i think you should use the battery ground as well… i am not sure leaving it off will allow for good charging… maybe pre-tin the top or the XIAO before sandwitching maybe do corner pins first to hold in place… as i am sure you know… if the copper is to the proper temperture the solder will flow … especially with the aid of flux so flux it… make sure you have an iron tip that can really get the heat into the oval place the iron tip thru the hole to heat the xiao pad and thru hole at the same time then apply solder and it should flow to the joint… Just my opinion