Best approach is one at a time.
For your pump, is the cover lose also? It doesnt make sense that the oring is smaller, but as long as it mates up to the pump trap, it should seal. Make sure also, that you're lubricating the oring (just not with vasaline - petrolium based lubes will tear down an oring).
Make sure there's no debris on the oring to allow the leak.

For your filter, you may have to tear it down to ensure that the sand filter laterals are good and seated properly, and not cracked or loose.
Try backwashing it a few times. Check your clean filter pressure and see how long it takes to build up 3-5 psi increase in pressure. That may indicate the need to replace your sand.
Sand blowing back into the pool is a sure sign of a filter lateral not seated or is cracked.
It may also be your multiport valve gasket needing replacement.

Not too familiar with the pool cleaner you have.

Hope this helps.