Converting polyline to polygon without 'Feature to Polygon'

Question

I am attempting to create a script tool to automate some processes at work. Part of this process was to manually cut (using Cut under Editor) a buffer. My approach to this has been to try and split the polygon instead by using the line layer that we were tracing. However you cannot split using a polyline AFAIK, and I would prefer to avoid using 'Feature to Polygon' as Advanced licences are limited at my workplace. For the same reason I am not sure if I can use the 'buffer to one side technique'.

I have been reading up on how to convert polylines to polygons and I have completed part of the process:

# Feature Class to Points
# 
# Paul Smith (2012) paul@neoncs.com.au

# Imports
import arcpy
import numpy
from arcpy import env

env.workspace = "U:/ArcGIS"

#Inputs from user parameters
InFc  = "U:/ArcGIS/Default.gdb/clipdefenses"
OutFc = "U:/ArcGIS/Default.gdb/clipdefpoly"

# Spatial reference of input feature class
SR = arcpy.Describe(InFc).spatialReference

# Create NumPy array from input feature class
array = arcpy.da.FeatureClassToNumPyArray(InFc,["SHAPE@XY"], 
spatial_reference=SR, explode_to_points=True)

# Check array and Exit if no features found
if array.size == 0:
arcpy.AddError(InFc + " has no features.")

# Create a new points feature class
else:
arcpy.da.NumPyArrayToFeatureClass(array, OutFc, ['SHAPE@XY'], SR)

So I have the points of my polyline, but I am unsure of the next steps to create a polygon out of them.