Hi Nick,
Regarding to your question, these versions are not same:
>>My LPC54018 SDK version 2.5.0, CMSIS: V5.1 , fsl_usart version V2.0.3
>>AWS demo CMSIS versiom : V5.0 , fsl_usart version V2.0.1
These version numbers are for each module. they are not same. it's normal. As long as they are all included under same version of SDK 2.5.0, they are compatible.
RTE_Device.h is real time environment file for CMSIS. It's only for CMSIS demo code.
To add other module to a certain project, I suggest you use "manage SDK component " feature.

But please note, adding/removing components could potentially break your project. You need be very carefully.
Have a great day,
Jun Zhang
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