You have hit a number of issues here with the eduappcenter and the LTI implementation by Instructure. Firstly, the URL you have treated as a launch URL is actually a URL to a configuration file which can be used by Canvas to configure the tool. The XML contains the launch URL amongst other properties. Whilst other LMSs support the configuration of LTI tools using XML, the format used by Instructure does not conform to the IMS spec and so is unlikely to be useful in any other system. And, in any case, Moodle does not support XML configuration.
Secondly, I take issue with Instructure calling all the apps listed in the eduappcenter as being LTI. The Trello example you selected is a good example. It does not require a consumer key and secret for the simple reason that it is not accessing a secure resource. When I launched it a publicly available video from Khan Academy was displayed in the browser. The only sense in which this is LTI is that it performs an LTI launch internally and redirects the user to a publicly available page - there is no passing of personal data to the tool provider, it is merely a convenience for Instructure because it has adopted LTI as the mechanism for adding any content to its courses. So when you see an application in the eduappcenter which does not have a key symbol ("Requires Consumer Key and Secret") then this is not an LTI app in the sense that there is a secure handoff to a third party application and the same end result could be achieved without LTI as access to the resource does not require any credentials.
If you want to try out an LTI app then, as you have realised, you just need three pieces of information: a launch URL, a consumer key (to identify you as a customer) and a consumer secret (to secure the messages between the systems). Here are some samples you can freely used without needing to wait for a key and secret to be issued by a tool provider:
A. LTI Test suite (http://lti.tools/test/tp.php, use any key with a secret of "secret")
B. Sample Rating app (generate a launch URL, key and secret by clicking the button at http://www.spvsoftwareproducts.com/php/rating/ - this provides access for 14 days)
C. Example integration for WordPress (generate a
launch URL, key and secret by clicking the button at http://www.spvsoftwareproducts.com/php/wordpress-lti/ - this provides access
for 14 days)
HTH.
