Are other Joomla users having problems with the Pay Later options?
I'm not going to say Joomla is a second rate citizen, but it is by far, the least used cms of the three big ones, by people that use CiviCRM. It is the only one I use on a regular basis, so don't get me wrong, I'm not talking it down. I think the answer though is basically going to come down to if you what the demo site updated, you are going to need to ask how you can update it for others to be able to use. I tend to find that, while there are a could of us that use Joomla a lot and try to support it, if I want an integration with Joomla or a feature or whatever, I need to own that and make it happen.

As for the pay later, I don't think I've used that in 4.7, but I do have it installed, so if I can test something out, I'd be happy to. even if you just wanted me to create a blank Joomla 3.7 (just dropped today) and install the latest CiviCRM for you to be able to test on, I'd be willing to do that. In MatterMost, you can reach me @rtobias81, or just comment here and eventually I'll get an alert.