I will try and type up a recap of what I have learned from all this, but in the short-term, be gentle on the tubes going through the firewall to the heater core.
Yes, they are secured directly to the heater core and "located" by rubber gaskets and plastic holders, but they are not robust.
I can see where simply trying to remove and existing heater hose off the tubes could cause a problem on an old or original heater core.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)