One of Many Mangen Posts

"How can you show me that things exist outside of the mind?"
This question implies that you believe that you and me are separate, and so another mind being outside of your mind, otherwise you do not carry any meaning with the words used in that sentence.
But besides that, I could only show you that things exist outside of the mind if you accepted a differentiation between the mind and things that are not the mind.
"Because there is no way and has never been any way to determine facts or truth without the use of our mind..."
This statement makes a claim that truth cannot be known. A claim is either true or false, and the only possible outcome for that statement is that it is false (truth can be known) since if truth cannot be known then that claim cannot be true. This is the result of the prior claim by Mangen "There is no objective truth." Truth is subjective. We have to decide if an idea is true based on our senses.
"...which we have proven time and again"
What do you mean when you say proof?
"...that we barely understand it"
How do you know that we -barely- understand it? What if we completely understand it, or not at all? You would need to differentiate between what is known and unknown to do that, which is going to require truth.
"Just an observation."
An observation of...?
"I imagine characters in my head all the time..."
Those characters are a creation of your will, or they are the result of chemical reactions outside of where your will emerges and is interacting with your will externally, a hallucination.
"I still have to interpret you through my mind."
You have to interpret, since his actions do not bend to your will, making him external to your mind.
"No because thats one of a great many hypothesis' and none of them could be the case."
None of them, is a claim. True or false.
"...we have deceived ourselves with words and concepts..."
Deception implies intent. So we intended to actively believe falsehood, to what end?
"...thinking we know the true state of the world when really all we have is highly subjective and extremely arbitrary experiences and measurements of the world."
'True state of the world' has to mean something for this to make any sense. We've already established that truth is subjective, so there shouldn't be a conflict. 'Arbitrary experiences' is non-cognitive. By definition an experience is not arbitrary.
"Now Im sure some modicum of truth can be found in that subjectivity, what that truth is I do not know..."
Again, truth has to mean something here. Not knowing what 'that truth is' misunderstands why anyone seeks truth.
"Im playing the game of extreme agnosticism right now. I dont take anything as a fact."
Fact is implied here to mean truth, and once again, has no meaning.
"But just because the human brain is incapable of conceiving things a certain way does not in fact make those things right."
Now this is a little bit better actually, as it looks like you are not making a claim about human brains but instead outlining a scenario where the brain is taken to not be capable and then suggesting something is necessarily the case as a result.
Still, what is suggested as necessarily the case is 'right' which seems like it would mean truth, but I'm not sure.
"The problem with cogito ergo sum is that thinking is not defined.."
I agree partially. It isn't a good way to prove existence, but because existence isn't defined properly, that's what leads Descertes down this path to begin with.
"I question being but am unable to communicate with you unless I use language that implies it."
To question being, you need to define 'being' first, and then differentiate it from not being.