> Israel for years had the means to completely exterminate Gaza many times over, but they never did
In a theoretical sense that Israel had the means to, sure. But in practice, Israel couldn't really do that without disastrous consequences to itself, could it? Having protection and support from major powers, like the UK and USA, has been a lifeline for Israel ever since its creation. If US stopped using its veto power to protect Israel within the UN Security Council, it would have been under heavy sanctions for a very long time now, and its prosperity and perhaps even existence would be under serious question (see [1] and [2]). If US stopped providing military funding and arms support to Israel, it would seriously impede its ability to wage war. If those same powers were not helping its defenses (either directly or via deterrence) against the many enemies it has historically had in the region (it has basically no neighbouring country that it is in good relations with), it is doubtful that it would still have existed to this day. The governments of the countries protecting Israel would not be able to keep doing that if Israel made a move so radical such as large-scale extermination of Palestinians in a post-WW2 world that does not look fondly on genocide. So no, they do not have the means to completely exterminate citizens of Gaza in practice.
And even if Israel had both the means and the freedom to completely exterminate Gazans yet chose not to do it, does NOT committing something so obviously atrocious provide any justification for doing other kinds of atrocities that most people would categorize as less severe? I strongly argue that it does not. To use a silly analogy, someone who stole a significant chunk of your money from you does not deserve praise for not stealing all of it just because they theoretically had the means to. They have still done you injustice, inflicted harm, and deserve condemnation. No one should be free from criticism when they commit acts that deserve criticism.
About your point (1), the reactionary expulsion of jewish citizens of the neighbouring states in the post-Palestinian-ethnic-cleansing-of-48 world [3] is also a tragedy that also deserves condemnation. It is not the current burning issue, however, while Gazans are being bombed and starved to death as we speak, which is why there are so many people talking about Gaza right now and not about that. Nevertheless, I would not want to stop anyone from talking about it, as again, no one should be free from criticism when they commit acts that deserve criticism, and I welcome anyone with time and motivation to open topics about it. This current topic here, however, deserves full attention of its own without whataboutism.
[1] https://en.wikipedia.org/wiki/List_of_United_Nations_resolut...
[2] https://www.middleeasteye.net/news/43-times-us-has-used-veto...
[3] https://en.wikipedia.org/wiki/Jewish_exodus_from_the_Muslim_...