how to integrate properties defined on multiple rows using a data.frame or data.table long format approach

If I understand OP correctly, you want smth like this:

dt[, {bigN = .N; .SD[, .N / bigN, by = subg]}, by = group]

or maybe (and very similarly) this:

dt[, {counts.sum = sum(counts); .SD[, counts / counts.sum, by = subg]},
     by = group]