1. Yes, they should provide alternate routing. It may be on SFO-LHR depending on expected load.
2. Seattle is a pretty compact airport. You’ll be flying into concourse a or b. If all things go according to plan, you should be able to get from your arriving flight to S terminals in time. Now, you’re flying out of SFO in June, so you’ve the fog. That’d be my one worry. I’d consider going on an earlier flight SFO-SEA if it’s not cost prohibitive. The Delta lounge at Concourse A is fabulous to waste an hour (not up to international lounge standards, but for us domestic types….)