How would I cut this wood perfectly straight?

We'll, I didn't read through the whole thread to understand the details fully, but I can tell you how a professional fence builder goes about things.

First they plot the fence corners. Then they set the corner posts, making sure they are centered and perfectly vertical with a level. Then they run a string from post to post and all the posts along the string using nothing more than a level. Once all the posts are set, the tips of the posts are trimmed.
 
So it seems like you are a part of a mission impossible, it seems,.... but a cheap mitre box and handsaw will cut the poles square assuming the poles are not bend

so perpendicular, would be called called a square cut

parallel would be a straight cut, in which case you need a table saw, and skills.

no skills = dont use a table saw
 
No. I own a chainsaw and a hand held saw lol
cut them to height with the chainsaw, just get close to square. Then set in concrete inside flower pot planters, top off with shiny glass beads. Attach tacky led lights and post on Pinterest. problem solved
 
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