Poland held out for almost 4 weeks against Germany in '39, yet France and Britain did not assault western Germany while it’s army was busy in Poland.
France and Britain’s plans for fighting on the western front involved trapping a German thrust into France through Belgium. (They were expecting the same strategy they saw in WW1, I guess.) This is how the sitzkrieg came to be.
I doubt that anything would have been better in '38, in regards to the FR/UK plan.
But- would & could Hitler actually invade the united Czechoslovakia? The appeasement policy was wrong as it handed Czechoslovakia over to the Germans who could just walk in, after the defences in the Sudatenland were gone. The Germans tanks still were armed with machineguns only* at that time, and there weren’t a lot of them.
The western allies assumed Germany had a formidable army. They felt that they needed to assume a defensive position behind fortified lines and wait for a German attack. (If Germany didn’t attack, they would grind Germany down by a naval blockade and aerial bombing.) Based on WWI, they figured the German attack would get ground up by this defense and once the German army was weakened, they could launch their attack into Germany. The western plan assumed there would be an offense into Germany at some point (although they probably hadn’t figured it would be postponed until 1945).
A Czechoslovakia campaign could have moved this schedule up. The battle in Czechoslovakia could have taken the place of the battle in Belgium in breaking the German army and the western powers would be able to launch their main offensive.
This obviously is assuming that the Czechs put up a credible fight against the Germans. The Poles fought bravely in 1939 but Poland collapsed quicker than the western allies had expected (they had assumed Poland would hold out through the winter). This reinforced the fears of Germany military might in London and Paris and made them more cautious. A German setback in Czechoslovakia would have had the opposite effect.
Cite? AFAIK Poland was expected to hold for two or three months, but that didn’t take into account Soviet invasion from East. Poles expected to hold for up to six months, but that plan included France and Britain attacking Germany, not just dropping leaflets.