It's important to understand the definitions of working relationships/agency laws in your market before allowing anyone to represent you, or potentially act on your behalf in a transaction/establish "procuring cause" by showing you a property. I inserted "potentially" because procuring cause being established simply by showing a property is something of a grey area, which I will address later. It's a grey area that can easily be avoided by not seeing a property with an agent that is not your agent, but I'll come back to that...
The listing agent represents the seller alone in this case, as they have signed a listing agreement in which they agreed to act solely on behalf of the seller and to promote the interests of the seller with the utmost good faith, loyalty and fidelity. The listing agent negotiates on behalf of and acts as an advocate for the seller only. I believe Texas is the same as Colorado, where dual agency is not permitted, so the listing agent couldn't represent both the seller and the buyer even if they wanted to. They would need to revert to a transaction broker/intermediary (like a ref instead of a coach in sports, with no fiduciary duty to and without advising either party). This would be tricky for them because they already have a relationship representing the seller and have already advised them in regards to the specific transaction.
So it's no surprise he doesn't want to present your offer without buyer representation, despite the temptation of being able to keep the whole commission (unlikely as his listing agreement/agency agreement/commission laws/broker probably prevents this). The likelihood of the deal going sideways is too high even if he were able to do this, and there would be too much liability for him to go down this road when he could work with a represented buyer and keep it a clean, above-board transaction.
Now regarding Susan's role: she does not represent the seller simply by working for the same brokerage as the listing agent, but it seems you view her being in the same brokerage as a potential conflict of interest. Whether or not that's true remains to be seen. Before making this judgment it would be fair to at least interview her and ask whether or not she has done transactions with fellow agents within her brokerage before, then maybe ask to speak with those clients in order to determine if she did a satisfactory job representing them. It's not uncommon for agents to work on the other side of a transaction with another agent within their brokerage, we are all independent after all, (many of us don't even know the majority of the other agents working in our brokerage) and I would not automatically expect her to provide sub-par representation just because she hangs her hat with the same brokerage with the listing agent. You could also inquire further about her relationship with the listing agent, or ask their broker to assign a different agent from their brokerage who would be more "arms length"/ not brought into the deal by the listing agent. As the buyer's agent, she would have to agree to act solely on behalf of you the buyer and to promote your interests only with the utmost good faith, loyalty and fidelity. The buyer's agent negotiates on behalf of and acts as an advocate for the buyer only. Since she showed you the property, she may feel like she has procuring cause at this point, which is when the actions of a real estate agent conclusively result in a property's sale. Simply showing a property typically does not do enough to technically establish procuring cause, but she may not know that, and she may feel entitled to a commission if you end up buying the property now, so the waters have been muddied.
The waters are even more muddy as it relates to the agent you have selling the other 2 buildings for you. What kind of agency agreement do you have with them? They probably would want to represent you in this purchase, as they are acting as your agent on two other deals, so they would probably appreciate your loyalty.
Basically it seems that you thought you were making things simpler by approaching the listing agent directly, but unfortunately it made things complicated. At this point I'd probably recommend having a frank conversation with both Susan and your other agent to see who you want to represent you in this transaction, and go from there.