Hello.
Once you enable Spanish usually you should run Load missing language data. This tool fill/copy all missed translation for example if some of the dynamic data not translated like Product description, Tax class name or description and more. You can do this manually up to you.
http://docs.abantecart.com/pages/localization/languages/edit_language.html

If you have purchased and installed Google Translate API extension with active Google API key (it is paid) you able not copy but auto translate with google api.

If you want to translate or correct manually in Language Definitions just do this for what you see is wrong translated, you dont need to check everything

Hello.

Please separate your questions to one question per post.

-1- How to correct or complete the partial translations on the frontend of the site? I mean some pages/parts of pages in the frontend still display English instead of the Spanish language I chose as default. Is there a way to show the whole site in Spanish mandatorily and no option of changing to/from English?

If you need to correct just a few text in spanish go to Admin > System > Localization > Language definitions.
In case you have a lot not translated definition better is to change this directly in the xml file you have in extensions/default_spanish/
There is also way to connect Google translate API but it is paid solution
http://docs.abantecart.com/pages/localization/language_definitions.html
http://docs.abantecart.com/pages/localization/auto_translate.html

Hello.
Check list of most important permissions here http://docs.abantecart.com/pages/install/options/regular_installation.html
then clear AbanteCart cache, and enable System Check in admin settings it will report you if some permissions are incorrect too

Hi.
Open browser console, reload page and see if some warnings in the console

Hi. So issue resolved right?

Hi.
Can you replicate it in few browsers or you see it only once? If you provide an url we will check your site from our end too 

Thank you, what is your site url?

Hello.

I think you missing documentation and replies to your posts 
http://forum.abantecart.com/index.php/topic,5578.msg24258.html#msg24258
http://docs.abantecart.com/pages/settings/stores.html

single domain note

If you use domain.com as main store, it will work as *.domain.com and other stores can not use same domain
If you need to use subdomain store1.domain.com as an additional store, please make sure main store use subdomain in settings (example: www.domain.com)

Try to add www