I can only answer your first question because I honestly can't think of an answer to your second question at this time.
To your first question:
"1. How do we know that the present climate distribution is optimal, and that a few degrees warmer would be a disaster? After all, large areas of Siberia and northern Canada would then become temperate."
Siberia and Northern Canada are Arctic climates. They depend on the ice that normally covers the area. If they become temperate climates, all the ice would melt, thus cause catastrophic flooding, not to mention rising ocean levels as a result. And, current temperate climates would become tropical climates...hence why I said New Jersey is becoming like Florida or any Gulf Coast state in my rant.
Warmer temperatures would mean larger and more intense storms, including tropical cyclones. A warmer planet would cause damage to many crops that we depend on for food and agriculture. But, that's just the tip of the iceberg. Plants and animals would die off...and, so would we in the long run. If that's not considered a disaster, I'd hate to know what you would consider a disaster.
Summers are definitely much longer than they used to be. Spring is almost non-existent with maybe a few weeks of it before Summer comes along and stays through much of Autumn. As I said in my rant above, Christmas is usually cold with average highs near 40. But, last Christmas felt like May at least with those 60s and 70s. Is that not unusual? We haven't had a White Christmas since the mid 1990s. And, if this keeps up, we'll never have another one again. Of course, warm weather lovers won't care about that. They'd probably say something like: "Oh, boo-hoo...no more White Christmases. Get over it."
Apparently, this seems to be the new "normal" ever since the 21st century began. While we didn't have Y2K, things did go from bad to worse. 9/11/2001. The 2004 tsunami. Earth off its axis. Significantly rising temperatures. Bigger storms. Devastating floods. Extreme droughts. If it's not global warming, then it's definitely something unusual that should not be occurring on this planet. Do we agree on this at least?