For each excel row from bottom up

Based on your inputs a regex could look like this:
grafik
with refering to Groups:
grafik

And above suggestion just need a small adaption

Assign Activitiy
dtResult = dtDataClone

Assign Activity
dtResult =

(From d in dtData.AsEnumerable
Let mts = System.Text.RegularExpressions.Regex.Matches(d("Items").toString, "([A-Z]) (\d+)")
From m in mts.Cast(Of Match)
Let ra = new Object(){d("ID"), m.Groups(1).Value, m.Groups(2).Value }
Select r = dtResult.Rows.Add(ra)).CopyToDataTable
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