1. Being able to move a star and moon is a very impressive feat of telekinitic strength. Celestia, Luna, and Discord are the only individuals shown to have moved such bodies without recieving some significant power up like Twilight at the season 4 finale. Such feats are considered extremely difficult to pull off in Equestria as outside of those three, it took the unicorn tribes strongest mages to pull it off, and they were losing their strength for it. If Celestia and Luna hadn't shown up, it would've likely resulted in some catastrophe once the unicorn tribe lost the means to move those solar bodies.
2. It was implied that Cadance lost to an unpowered Queen Chrysalis considering where she was situated at and can presume it happened before she started siphoning up Shining armor's love for Cadance. It took a powered up Chrysalis to beat Celestia, which she was even shocked she won. Without such a power up, the comics show Celestia stomps Chrysalis as she caused those holes you see in her legs when she originally didn't had them (though why her subjects do is a case of writer failing to note that) and Celestia actually imprisoned her and her changeling army inside a volcano. From the recent seige of the crystal empire arc, both Luna and Twilight (heck, Twilight defeated Chrysalis as a unicorn in the comics) were shown to be able to beat an unpowerd Chrysalis on their own so even more evidence to suggest Cadance is the weakest of the bunch.
To be fair, even the weakest Alicorn Princess is nothing to scoff at as she did held a city wide shield spell for an undetermined amount of time without rest against a powerful malevolent being in Sombra and does posses some powerful transmutation hacks as she turned water into crystals in Princess Spike and we can presume she at least has all the abilities Pre Alicorn Twilight and Trixie with Alicorn amulet possessed and shown via power scaling.
Hope that helps explain why Cadance is conisidered the weakest of the Alicorn princesses.
As for your other point, I agree the Crystal Heart is extremely powerful if it was able to hold a deadly blizzard all this time and defeat Sombra as well.