The answer to the latter part of your question: only the INITAL RVR may be replaced by pilot assessment. Thus if reported 300/300/300 and you need 400 you cannot go. Had the report been 300/400/400 you possibly could.
In addition, you could assess the first RVR and determine the distance required and if subsequent RVRs need be assessed. Think of a 10,000ft strip and you require just 3,500 to get airborne, the latter RVR for sure can then be discounted, quite legally, provided operator manual allows.
Regarding regulations, AMC and CS refer to different areas and may or not overlap somehow. In this case the regulations stating 400m required without centreline lights prevail.
Not having the regulation in front, I am unable and unwilling to comment more detailed, but be careful that some regulations are written such that you need to tread carefully to ensure all variables are complied with.