The project is currently in maintenance-only mode because it is basically finished and I have other priorities (most of the updates are about modifying the set of supported sources).

There is no officially supported way to import/export via a config file, but you could use the developer tools of the browser to directly read / modify the preferences.

To debug an extension, see the guide at https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Debugging

After opening a debugger for the extension's background page via about:debugging (see the above link), you can interact with the storage via the chrome.storage.local API, as documented at https://developer.mozilla.org/en-US/Add-ons/WebExtensions/API/storage/local

For example, to convert the storage to something that you can copy and paste in a file, run the following from the debugger's console:
chrome.storage.local.get(null, items => {
console.log( JSON.stringify(items, null, 4) );
});

You can then manually edit the configuration in the file (at your own risk), and save the changes back as follows:

chrome.storage.local.set();

For example:

chrome.storage.local.set({
"panelOffsets.youtube": {
"top": 100,
"right": 204,
"width": 275,
"maxHeight": 600
}
});

I have published an update (3.14.1) which allows you to permanently hide the in-page button.

To disable the button in the page, right-click on the extension button in the address bar and toggle the "Show Lyrics Here button in the page." menu item (this instruction also appears in the tooltip of the in-page Lyrics Here button).

font-weight sounds a bit too specialized to warrant a dedicated option.

You can use a user style manager such as Stylish to change the font-weight of the elements if you wish. Almost every significant element in my panel has a recognizable class name that you can select with CSS. These class names can be found by right-clicking on the lyrics panel and choosing Inspect element (to open Firefox's developer tools).