1.  formamide and/or urea most likely. They both lower the melting point of the ss DNA . This allows the structures to unravel at lower temperatures. Typically you want to reach for  melting temperatures around 50° C, and then run the gels at that temperature. RNA is often denatured with harsher agents, like formaldehyd because RNA form stronger structures. 

2.ds DNA can exhibit supercooling which is a very elegant method of packaging DNA. This is a huge topic and mathematically derived. It's important is mostly due to it's role in gene expression via the interaction ds DNA has with proteins it wraps around-his tones. 

Note that ds supercooled DNA forms two structures; a plectoneme or a toroid, and can also have combinations of these two. It's interesting stuff. Google "DNA supercooling" with an image search  and look for a figure that shows the different levels of winding and wrapping:

I'm not very familiar with this source but the figure looks to be on the up-and-up.
https://www.google.com/search?client=safari&hl=en&biw=320&bih=460&tbm=isch&sa=1&ei=z_KTU4HAEsqTqgbakYHACA&q=dna+supercoil+hostone&oq=dna+supercoil+hostone&gs_l=mobile-gws-serp.3..30i10.30421.35370.0.35846.13.12.1.0.0.2.489.3028.0j6j3j1j2.12.0....0...1c.1.45.mobile-gws-serp..7.6.975.GZlKkpdiujQ#facrc=_&imgrc=mfy6XWXCf9A4sM%253A%3BMNOv-N10Zf4moM%3Bhttp%253A%252F%252Fclick4biology.info%252Fc4b%252F7%252Fimages%252F7.1%252FDNAfibre.gif%3Bhttp%253A%252F%252Fclick4biology.info%252Fc4b%252F7%252Fpro7.1.htm%3B250%3B250

Good question