Is there any mean to add some script or pre validation plugin that will make a lookup to be the multi select. I don't want partylist as I am not working on activities also I don't want N:N relation solution as my client has some concerns on it.
No, it's not possible to create a lookup field with multi-select.
This because lookups point only to a single entity, also if you hack the control with JavaSript in order to allow the multi-select, CRM DB has not the field to store the selected values.