To get the text label from a current source you need to use a little Lua.
Helm can return the entire list of router names using the InputNames property of the router. If we split that into a table, we can then look up the source number to convert it to a name.

--lua
 out1_src = helm.getRouterSource("MY_ROUTER","0","0"); -- get the source
 -- construct the names table from the InputNames property - this could be cached for efficiency??
 names={} -- create an empty table

for k in string.gmatch(H.VIDEOHUB_1.InputNames, "[^%c]+") do
 names[#names+1] = k
 end
 -- look up the source in the names table
 H.LABEL_1.Text = names[tonumber(out1_src)+1]

Best to put the table construction in the panels OnStartup event so the names table is only built once per panel run.
The rest can go in the router OnTallyChange event.