I’m not sure “error” is the right word. “Error” usually connotes something objectively incorrect, arrived at by mistake—like getting the wrong answer to a math problem, or thinking “you can’t get pregnant your first time.” The things you describe aren’t “errors”—they may be poor writing or bad drama, but it’s not “objectively incorrect” to kill off Icheb
That said, one of the strengths of Picard is that it takes two plot elements that, to my understanding, were largely poorly-received across the fan base, the death of Data and the destruction of Romulus, and it builds a story around them. Whether that story is any good is sorta beside the point. Future writers could do something similar with your objections by taking plot elements that are disliked and attempting to spin a worthwhile story out of them.
with respect to a couple of your specific objections:
1) How did VOY show the UFP was “enlightened” as to former Borg? Granted, it showed the crew of one small, isolated starship *gradually* coming to trust Seven and a small handful of other individuals during the course of daily contact. It’s hard to make an “other” out of someone you work with each day in stellar cartography. What about the vast majority of UFP citizens who never have that opportunity? What about the fact that this enlightened UFP was also sending obsolete EMHs to work in what were essentially slave labor camps. Also keep in mind that this statement was made after a major disaster on Mars which was blamed on artificial life forms. Ask a Sikh in the United States how they felt in the US after 9/11. They had nothing to do with the attacks and were from a completely different religion, but because of superficial similarities were subjected to anti-Muslim violence. Is it that hard to imagine xB’s being lumped in with “the rest of those artificial sorts” after a society-altering disaster?
2) was Data isolated in a server? I think the show very intentionally left it vague as to whether Picard was somehow interacting with the true consciousness of Data or if it was all internal to his own dying brain. The truth doesn’t actually matter here. The point is Picard finally coming to peace with the loss of his friend.