Say Hitler dies after occupying the rest of Czechoslovakia or decides to hold off on demanding Danzig or whatever.
Could Italy invade Yugoslavia during the early 1940s without causing Britain and France to the enter the war.
Yes. Yugoslavia wasn't an ally of either. The little Entente was defunct. Romania wouldn't help Yugoslavia, while Hungary would probably jump in, and Germany and Bulgaria, with regard to their respective border regions, might behave like Poland with regard to Cieszyn.
France and Britain would enact worse sanctions than in the Ethiopian case, virtually severing the link with that Italian colony, and reducing the Italian economy to shambles. But they wouldn't go to war, especially not with Germany on the back burner.
There are two problems, though:
1. Mussolini's main strategy was opportunism. If Germany has gone non-aggressive and there isn't another war going on, he's very unlikely to attack Yugoslavia.
2. Italy can attack Yugoslavia from two directions (including Albania) and from the sea, but even so it's rough going as to the terrain and it's where the Yugoslavians are waiting. This makes the probable and possible help from other directions, mentioned above, more important. Another very useful contributing factor would be a Croatian insurrection, which would also contribute in terms of a casus belli, and would disrupt the Yugoslavian situation, mobilization and resupply. Without either those other powers helping, or (preferably and) upheaval in Zagreb, the Italian offensives might well peter out after limited advances on both fronts.