So currently I have a mock-up that has a single fuse and a single switch on the AC load line. Is this safe?
The majority of devices found in the home or office are wired this way and there are few issues. The problem is that if you live in a land of non-polarised mains plugs which when reversed, can make the non-switched wire live, then the internal circuitry remains live even when the switch is off. A table lamp, for example, has a single-pole swtich and the terminals are exposed when the lamp is removed. Your setup should be much safer if boxed up properly.
Would it be better to use a 2 pole switch to switch in both load an neutral? Why?
When the switch is off both lines are cut and the circuit is completely dead.
Should I fuse each SMPS separately? If yes, why and where?
Check your SMPS and you should find that there is a fuse on the mains side of each. The main fuse protects the wiring inside your unit in the event of a fault. Your SMPS units will supply a total of about 100 W so will draw a max of 130 to 150 W fully loaded. In 110 V land this will be about 1.5 A and in 230 V land it will be less than 1 A. Use a 2 A or 1 A fuse as appropriate in the common live line, point 'A' as you have correctly shown. This will limit any fault current to a much lower and safer value than your mains wiring can provide.