In philosophy, terms can often have fuzzy semantics. A term might be more meaningful when created, but as the term gets scrutinized, people often use slightly different meanings to the term that more easily supports their point, but in the process the term becomes less meaningful.
In the case of humanism, humanism originally meant a code of ethics *centered* on benefiting humans. This is incompatible with sustainability, because in decisions where one must decide between sustainability and the benefit of a human, the benefit of the human would win out. Afterwards, apologists have changed the definition of humanism to be something synonymous with rationalism or liberalism. Although this new definition isn't too useful at distinguishing anything (you can just use the word rationalism or liberalism instead), it is at least compatible with sustainability.
As far as idealism is concerned, I believe the commonly held meaning of idealism is: allowing an ideal or value take precedence over pragmatic need. Since sustainability is often opposed to what some people consider pragmatic. I believe idealism is definitely compatible with sustainability.
Yes, the tenants of sustainability go against those of humanism. While sustainability focuses on the continued use of products and their affect on the planet, humanism is only concerned with the wants and needs of human. Humans take whatever they want from the planet and have little concern with the Earth's health. Idealism is just that, ideal. There is no possible way to achieve an ideal status.
I do not think that sustainability is compatible with the principles and idealogies of humanism and idealism. I do not see how it can be the same. Sustainability for example is based on the principle of making sure that the environment can find a balance with humanity. But there is no way that is possible with the idea of humanism.
Secular humanism (which embraces human reason over faith or inherited wisdom) is almost always associated with more liberal causes. Sustainability (the belief that we should focus on reducing our environmental impact, even at the cost of material wealth) is generally regarded as a liberal ideal. Not only are the two beliefs compatible, they are generally considered to go hand-in-hand.
No, I do not think that sustainability is incompatible with humanism and idealism, because we want to have peaceful lives that are sustainable for our children. People really do care about others and about the future. We can live the way we deliberately choose to while we sustain the world for our children.
Humanism is typically defined as having anything to do with humans and generally refers to movements within humanity. Idealism is defined as following unrealistic ideals. While sustainability is generally a term associated with ecological ideals that we should seek to meet. Humanism is a broad term, so to say the two are incompatible is a misnomer. Idealism may be a term we could use in relation to some peoples ideas on how to obtain sustainability. I don't think we can take these terms and say they are incompatible.