Thanks for using NextGEN Gallery Optimizer!
Would you have a "file not writable" error notification on your plugins page in the admin, at all? If so, please use its suggestions to help you modify permissions on Fancybox's css file. After that, simply refresh your plugins page and your nav arrows and the X close image should display as intended.
In regards to changing the title display in Fancybox, please see the following post...
You'll want to set
'titlePosition':'inside' to achieve the style you're after in your link. It's also centered by default.
And finally, in regards to manipulating Fancybox's descriptions...yes, it's definitely possible. Fancybox uses NextGEN's "Description" fields at Gallery --> Manage Gallery --> "your_gallery_name" for the lightbox titles, and they can be fully-formatted using HTML and CSS. In fact, you can even include links! For example...
This is an <i>awesome</i> photo of my cat. <b>RAWR!!!</b> To purchase, please <a href="http://www.mysite.com/purchase-my-awesome-cat/" style="color: red;">click here</a> (cat not included).
Hope this helps!