Well, I already ran it, and I disagree. I think it proved to me that it wasn't "impossible" but at the same time showed me how it was more difficult than intended and why the players were frustrated with it.
Note that the players already had two weeks to think about it as the previous session they had done all of the intelligence gathering, then tried soloing the scenario, and then rewound time to let them have a do over with the full time.
I also used the exact same approach that the party did; as I agree that simply doing something else using my superior knowledge wouldn't prove anything. As a rule, I always balance encounters assuming a direct approach as 90% of the time that is what the party will use.
I am not quite sure I agree with the idea that a single player is better than a time. I hear it often, but I have trouble believing that communication is so bad in the average group that it completely undoes all of the advantages of having five people working together to come up with and analyze solutions and to notice things. I actually started a thread about this a few months ago.
But yeah, if the group's communication really is that bad, then they really need to work on their teamwork; which doesn't actually do anything to disprove the premise that the scenario isn't impossible the players are just making tactical mistakes.
Old west.
Those long range shots are seldom made under battlefield conditions, they are generally made against stationary targets. And there have only been ~10 recorded kills at ranges of over a mile in human history, hardly reliable.
The problem with that strategy is finding clear lines of fire and then managing his escape afterward. If they had, say, put him up on a cliff face, he would have been able to shoot the reinforcements; but he would have been suffering a range penalty and been completely unable to assist with anyone inside the pyramid, and they further would have had to come up with a plan to retreive him afterwards. It wouldn't have been a bad strategy, but it was one with some drawbacks and at the end of the day it wasn't the one that the party went with.
The character is built as a cavalry soldier, and his backstory is that he was a dragoon in the military. He isn't really a sniper or gunslinger, rather he is built to "kite" enemies at long range. Which is a fine build (although I would personally give him some sort of armor and backup close combat weapon), just one that is far better on open battlefields than in undercover urban missions.
I do use the warped space explanation in my personal setting.
But the horror is still present as PC's minds can be copied through other means; some of them naturally occurring due to the nature of reality and time travel.