If we are affected, we can observe these effects.
No, it literally does mean that nothing else can affect us.
Absolutely not. We know only what we have observed and deduced in the limited life of the human race. Other things that we have not observed or deduced yet will emerge. But until they do, we don't know that they are there.
The problem is, that as far as can be determined, what we have observed, is all that mankind will ever reasonably be able to observe on this matter. So unlike the glass of water above, its like observing 99% of the lake and finding no lochness monster, then concluding there is no lochness monster.
What we have observed is what we can observe, and it may be all that we can ever observe. It does not mean that there is nothing else that can affect us. A better example is of someone stuck in a little cove, observing the ocean from that viewpoint, hence seeing only a fraction of it. As far as that person is concerned there need be nothing else anywhere.
Flat does not imply "infinite". I see no reason that it is "likely" the universe is "infinite". "Infinite" in actualized physical terms is meaningless,....
'Infinite' in mathematics is not a meaningless concept, hence the possibilities of applying that maths to the physical universe remain. You probably mean that in your model of things there is no place for this concept.
I have no mod points so I can comment only.