What I was expecting from a multiselect behaviour is to behave just as normal as long as no special key is pressed. I mean, if you have a row selected and click on another
jqGrid has several selection strategy, all using multiselect:true. To demonstrate there I created three example:
multiselect:true: http://www.ok-soft-gmbh.com/jqGrid/DataToMultiSelect2.htm. This is standard behavior which you not like.multiboxonly:true: http://www.ok-soft-gmbh.com/jqGrid/DataToMultiSelect3.htm. It seems to me that this behavior is what you need.multikey:"ctrlKey": http://www.ok-soft-gmbh.com/jqGrid/DataToMultiSelect4.htm. In the case the row will be selected or deselected only if Ctrl are pressed.If I understand your question correctly you should define both multiselect:true and multiboxonly:true to receive the behavior which you like.
If I misunderstood your question and you meaned something other please describe this in other words based on on of the above examples.