1. The font list provided by MS is very limited and lacks style... Is there any way I can use other fonts that are installed
on my computer (downloaded from various sites), and make them available for designing my desktop, icon text, folder text etc.?

2. The Wise registry cleaner informs me of numerous registry errors that are not safe to fix. If that's the case, then this information is useless to me, isn't it? If i can't fix it, what can i do? Should i fix anyway? Ignore and hope for the best...?

1. Put all font files in your font folder(c:\windows\fonts\), that restart your pc.
2. Under normal circumstances you can try to repair all the problems, including those marked as unsafe entries (I fixed all problems every time, and I have never experienced any problems in my computer). Howevery, because some software registered some deliberately incomplete COM information for specific purposes, although such software is rare. After removed or modified these entries, may lead to these software can not normal use. So for safety's sake, WRC make these appears incomplete or erroneous information of COM (entries) to as not safe completely.

You can definitely insert the fonts and also use it.
You have first download the different fonts which you want from search engine and store it into c:\windows\fonts\