In answer to your questions:
1) Custom forms are not attributed specifically to plans, just categories, so it would require customization on your part if you were looking to change this functionality. Our themes are not encrypted, so you are free to make modifications if you wish.
2) Twitter and Facebook fields appear on the default listing form, and whilst you can use the custom form to add more fields, it would again require customization to add more fields that would insert the social media icon as the existing ones are.
3) There is a default image for listings placed with no image, so you could replace that in the theme files with your logo, but if you want the logo for all listings, this would require customization.
4) There is already a contact listing owner button on the listing form for that purpose, and whilst you could create a custom field for this, it makes it easier for spammers/people looking to harvest publicly listed email addresses so is not ideal.
5) Through customization, yes this is possible. Thanks.