Your Swiss tourist sounds as plausible as any British tourist to Switzerland. If she goes into France, then up France and across the Channel, I would expect no problems. If your plot needs a difficulty at a border, send her through Germany first. Either the Germans can suspect she is spying on their railyards for the British, or the French can suspect her of being a German agent.

For a real-life "you are spies!" Border crossing in Eastern Europe a few years earlier, there's a later chapter in My Sister Eileen by Ruth McKenney.

The botanist I find difficult. If he were staying for an important conference then flew to meet his team in Singapore, more plausible. But botanists going into SEA jungle (I assume) carry a literal ton of stuff for logging, preserving, and maybe photographing specimens, not to mention their camping gear and food and medical supplies. Air travel was expensive, for the person travelling light. The route for his team and the gear, w/wo him, would be across the Channel, down France, a steamer from Marseilles through the Suez to Bombay, train across India to Calcutta, and another steamer to Singapore. Someone might remark on the similarity to Phileas Fogg in Around the World in 80 Days.

Hope thathelps! I've been researching the Interwar a lot, picking up period travel books at this stage. I have a 1937 home medical reference if you need something from it.