First let's establish that a List Collector variable stores a comma-separated list of sys_ids, and this list changes each time a selection/value is added or removed. So you must have an onChange Catalog Client Script to handle adding rows to the MRVS when this variable changes. Ideally, what you would do in this script is just clear the MRVS every time and re-build it with the contents of the new value - likely the same way you are automatically adding rows now. If you are unable to do this because users could have modified a row after it was automatically added, then the alternative approach is to get the value of the MRVS and loop through it to rebuild it with the existing values if a certain MRVS variable value matches a value in the List Collector variable value.
Give the first approach a shot if that sounds like it would work for you, and post the script if you get stuck.
