> The combo box would allow to make multiple selection
That's not how comboboxes work. They select ONE item. If you want multi-select, consider a listbox. And no, do not use a multivalue combobox, nor any MVF.
A listbox would also solve the problem of unselecting all.
One alternative to a listbox is a subform with an additional IsSelected column (Yes/No field, checkbox control) which is bound to a local temp table. This would enable to you join the query with this table to return only the selected items.
Re your field naming convention: it is a best practice not to use any spaces or other funny characters. They may cause notational difficulties.
Wow, PositionID is the same thing as ValHcm? You may want to reconsider your names again.
Feel free to ask follow up questions if something in this answer is not clear. I'm not exactly sure of your level of expertise.