Which type of entity should I use for editable homepage content? I have a set of social media links that I want to display in both the header and the footer, but I don't know where to put this content so that it is easily editable by users.

Right now I am using Panels and Views, but the links would be in the page.tpl.php, which has the header and footer areas.

I am thinking I could make a block, but it looks like you need to know the ID of a block to render it.

Is there a way to add a node, block, content pane, or mini panel and render it inside page.tpl.php?
Sure, there are modules that let you setup "blocks" of service badge/icon widgets for like Twitter, FB, YT, etc.

Look at Widgets, Social Media and Service Links modules.

I've used Social Media (which relies on Widgets) to put a block of badges on most pages via Context. And also to create other badge collection tokens that i've used within Footer blocks and embedded into WYSIWYG entries to show a FB link on the homepage for instance, or a mini-block of service links on a newsletter.