Hi,
0.3A to 8A, quite an achievement
Hope this helps...
Have you got a schematic and a photo of the actual circuit, thanks.
1) I see no reason why you shouldn't replace 154 with 155. WIthout knowing how the power supply operates (i.e. Is it an SMPS that operates at a higher frequency than the diode, and would that be relevant to the diode location and function in the circuit?)
2) Without seeing a schematic it's pointless conjecture. Maybe a shorted capacitor, maybe a shorted transistor, maybe a fault detector shuts off as there's a diode missing, ...maybe many things.
3) No, higher voltage limit isn't a problem.