Kill the power to the pump so the timer doesn't turn it on while you're still fussing with things.
Turn the multiport to Rinse and see if that will let some air creep in and the water in the filter to drain back out through the pump. If it doesn't, then set it back to filter and just loosen the drain plug so the drip turns to a trickle and then go do something else for an hour or two. Then when you do remove the drain plug, you won't lose but a handful of sand.
If that plug is standard pipe threads, your nearest hardware store should have something that fits. I wouldn't even bother trying to glue things.