Batteries, pumped hydro, thermal storage, and many other technologies exist. They just need to become cheap enough. Thus, the problem is once again price.
The engineering issues are in getting this stuff to be sufficiently cost effective that we can afford to put them into wide use.
Not all of these questions have have hard or problematic answers (e.g. especially the last one is probably much worse for nuclear), but they do need to be answered, at all. We've been answering these questions for nuclear for the past 50 years.
No, you won't run an aluminium smelter with them, but for load balancing during those (pretty rare!) times when you don't have either solar or wind they are fine.
I don't know about the temperature problem. I know they are used here in Australia at 40C+ and in Africa in similar climates.
I think that they are actually better for industrial use than home use. They aren't silent, and they need to be cycled. The noise isn't an issue in industry, and in home use I know the power cycling can sometimes be a problem because it is hard to find somewhere to dump the power (currently the solution is to dump it to heat, which isn't great - or alert the homeowner and get them to run a pool pump or something).
RedFlow (one manufacture) is in the same building as me, and they have an interesting report: http://redflow.com/wp-content/uploads/2015/06/Diesel-Runtime...
https://en.m.wikipedia.org/wiki/Flow_battery are cheap to produce, have decent power density and are solving this exact problem now in Africa and South America.
Production levels are just starting to ramp up now.
Yes, but we don't have that solution.