I don't see the problem with a HC for negotiations ... 11/14 days you can easily negotiate your first 10 gbg encounters, it would cost next to nothing and if the rewards are similar to HC it would definitely be worth it. 2 of those down days you can easily negotiate the cheap encounters in GE lvl 1/2, there's no reason to rush through GE. And for the last day, you have the option to collect on monday and negotiate twice on tuesday as mentioned, if you're that dedicated to the game that you want to optimize everything, you shouldn't have a problem figuring out how to do that. Of course, this is assuming you're a lower age player who bought the goods. Those in SAM+ who got the goods for themselves can easily use the excavation sites for any missing negotiations. So Inno has made the building easy to use for those who can actually get the goods for themselves. If you want to get it early you have to make adjustments, don't see an issue with that.
You already change your gameplay based on other great buildings (HC, AM, Dyn/Voyager, etc..), what's the issue with another one? There's nothing forcing a fighter to get one, but why shouldn't negotiators have a fun building as well? And if you really are unable to figure out a way to use it on mondays ...so what if you miss out on 7% of the collections? A 93% efficient HC would still be great.
Not everything has to be 100% optimized. It's a game, players can play it as they see fit. If you're a fighter you can get HC, if you're a negotiator you can get this new building. If you want both, great! I don't see the issue with options. There are plenty of buildings I don't have that others find very useful, I don't see anything wrong with that.
This is coming from a fighter, I hardly do any negotiations. I would definitely get this new building if the rewards are the same as a HC.