I get crumbs in our keyboard, from other users- not only me..., they don't usually cause a problem unless it's a big one blocking the key action..
I have had to rinse mine out a few times before also, most times it works as long as it dries fully before use.
I have a spare kb on hand just in case, the onscreen keyboard would be too much mouse work for my hands/arms.

yours probably just got some worn out internals or bad connectors inside.

nice that they are sending a new replacement for you.