Hello,
(1) As your sloc is HU managed. the left over should be transferred to IM managed sloc. Open space you can call it as IM managed sloc where there is no SUs and no warehouse attached. In other word, with reference to your above example, 40 CRT leftover in warehouse should be transferred to IM managed sloc using HUMO i.e. it should transferred to your open space. As you said above , "the SU is gone, the pallet label is gone..." you have to do it in system by deleting HUs in open space with Transaction code HUMO. Now in open space floor there is no HUs/SUs and you are free to add the stock. You can also transfer the open space stock which is IM managed to HU managed sloc via packing through transaction code HU02.
(2) In your above explanation you said that " When I try to add to existing stock (the 40 CRT) .... I can't because it's SU managed and the bin already has one SU"----> Actually it is possible to add the stock from one SU to another SU using LT01. In this case, the stock from supplying HU from where you are adding the stock will go away and the floor SU will contains original stock+ added stock.
Regards,
Milind Chaudhari