It's a nearly impossible situation.
Hypothatically, you could sign Markstrom and hold on to Demko and Seattle takes someone else in the expansion draft. You could trade Demko and he could go on to win the Conn Smythe, or he could have his game come apart like Schneider. You could sign Markstrom and trade Demko, but Markstrom declines after this season. You just never know. Goalies are the hardest position to predict. Rarely is there a sure bet.
Personally, I lean towards signing Markstrom (as long as the deal is affordable enough) and then let the chips fall where they may with the expansion draft. Rarely can you get full value for a goalie on the trade market.