The anomalies previous to that (created by future us, not "them") will have been first attempts to communicate. They could also have been unsuccessful first small tests (future "we" might still be good old typical "we" in a sense that those evolved people might still need a few first tests, not necessarily all succesful, before a procedure works...) The anomalies occured before the worm hole showed up. (Just like if a friend of yours would be living in a 2D world and a line there needs a swirl for a certain reason visible only from your 3D perspective, then when you draw the swirl on the 2D paper and it's messed-up, you draw it again, regardless if you're 3D and have a super futuristic looking pen - the swirl was too small, you do that again.)
Plus, you write: "Later, in the tesseract, Cooper assumes that they aren’t able to communicate via gravitation, and that’s why they built the tesseract in a way that can be grasped by Cooper, so that he is able to communicate." - AFAIU, their actual problem is that they just don't know how exactly to communicate in a way that could be understood in our 3D-world in that time.
He says, they have access to infinite time and infinite space, they just don't know where to dock on as they have no own emotional connection to their ancestors (and honestly - how much emotionally connected do you feel to a mesolithic skeleton). Remember they just built the tesseract. A tesseract, no more no less. The rooms, the exact moments are "found" by cooper (there is a line where he expresses that, he talks about finding a certain time... he finds the dusty room right when - after morsing "STAY" - he realises that he must indeed find the NASA facility to be able to communicate the data). Like if using a formula, the formula creating -a- tesseract that can contain infinite times of a certain space, and the emotionally bound person being the one that can fit in the values.
His exact words are: "They have access to infinite time and space, but they're not bound by anything! They can't find a specific place in time! They can't communicate." - I don't read the "They can't communicate" as that they can't communicate at all but the communication with us [for direct submitting of the missing data for example, by the future "we"] does not work as they can't dock on in specific times and spaces due to the missing "quantifiable" emotional connection. (Although I'd be very interested in learning details from Coop how exactly he "quantifies" his love for Murph on Sunday in relation to his love for Murph on Saturday but let's just leave it at that.)
In order to submit the data you need to know which data (TARS has them, gladly, but without him? Which exact data did these ancestors need, how to know after so many evolutions?), and even when you have that - communicate with whom? Cooper says "they chose her", but where? Where between those integrally "infinite" moments of her life, which of those infinite spaces to choose? And what language to use? C#? Google translate? Smoke signals? And who should translate them, how after so many evolutions?
I don't see an impossibility of communication at all, just the missing means to communicate with pre-evolved minds in a 3d world of that time.