etotheipi
Wow 
To try to begin to understand your method I drew a picture, I wondered if it agrees with what you had in mind (or whether it's completely off
)? The red line is the worldline of the charge, and at ##t' = 0##, its 4-velocity is parallel to the ##t'## axis, so the charge is instantaneously at rest at this time in ##S'##. Also, the vector ##R## joining this event to ##E## is null, and thus parallel to the yellow lightcones.
How does that look?
To try to begin to understand your method I drew a picture, I wondered if it agrees with what you had in mind (or whether it's completely off
How does that look?