Compression in Drupal 7
A start:

1) Yes.

Note: If I really want to know what is going over the line, I use fiddler (Windows).

2) See function drupal_build_css_cache() in common.inc (line 3603). You cannot influence the level but you can influence whether it compresses or not with variable 'css_gzip_compression', see default.settings.php line 432.

BTW: You do not want to influence the compression level: for compress once, use many cases, maximum compression is always the best. Transfer time and decompressing time at the client is linear with compressed document size. So a 1-time slightly increased performance hit will save a lot of network activity and client side processing in the future.

3) -

4) -

5) Compression will be better as it will be done only once and it will be done better versus mod_deflate that will compress on each request and therefore will use a lower compression level. I don't know much about mod_deflate, but I guess that if there is already a  Content-Encoding header that tells the content is compressed, mod_deflate should not touch it. But I have had cases where I got a backup file that was compressed twice, so mod_deflate might not be that smart and should  explicitly be told to do so.

6) There is no point, but there might be for other requests, especially dynamic ones, i.e. non cacheable pages. Note that only aggregated css and js and cached pages (variable 'page_compression') are compressed by Drupal. Non cacheable pages are not compressed by Drupal.