Joseph, I agree that the precise point at which it would be wrong to evict a guest from your seat in Shul is open to debate. Your opinion that this is at ברוך שאמר is certainly valid, although I disagree. In any case, I think this is a footnote to the main subject of this thread. You apparently agree with the basic premise that there is nothing inherently wrong with asking a guest to vacate your seat, provided that this is done in an appropriate manner and at a reasonable time, whatever definitions you would assign those terms.
I do not believe in forcing guests to “stand until Borchu.” Guests should be welcomed by whoever sees them, and guided to available seats as soon as they come to Shul. If this is not done for any reason, a guest should find an open seat and ask whether it is available.
In answer to your question, “If someone asks a guest to move in your Shul, who is responsible for finding him a new seat — especially if by time the regular guy finishes his coffee and arrives just before שׁוֹכֵן עַד there are no more other seats?”:
I think it is obvious that basic human decency requires that the member who evicted the guest should try to help find him a seat. Likewise, it is an entry level responsibility of ואהבת לריעך כמוך to help any guest in Shul find a seat, regardless of who forced him out of their seat.
Your implied derision of the “regular guy finish[ing] his coffee and arriv[ing] just before שוכן עד,” I think, is unnecessarily judgmental. The responsibility to make one’s co-congregants feel comfortable does not end with guests, or with helping to find seats. It certainly includes having a welcoming and accepting attitude.