completely cut section out from Song

That's easy, as long as you know were to find it. (As always in Reaper!) :) Make a time selection on that area, right click inside the selection (on free space - not! on the item) and choose from context menu "remove contents of selection (moving later items)". That's it.

If you struggle in such situations, try to use the "actions list". Reaper doesn't work like other DAWs. Its a huge library of scripts and actions. Every menu item also can be found inside this action list. And usually they are named pretty well to find them quickly. But it also drives you crazy if simple tasks don't show up intuitively. I know. :) There are rumors its not allowed to discuss such things. ;)

haha, ok ;) that worked out nicely... analogous, I also have the need to insert a complete new song into our arrangement, but I guess this will work with a simple "open song, CTRL-A CTRL-C insert at cursor"?