You are correct, when the modem wireless is disabled there is no way for you to access the device without a direct wire connection. Unless you configure both wireless to be active on different channels and providing different NAT IP ranges. Unfortunately you would still have to disconnect from your routers wireless and then connect to the Pace wireless to get into the device.
As far as differences to speed tests are concerned you will get different responses based on current usage of the speed test site itself and its location. The further away the more likely to have a higher ping response and slower speeds reported. They are an approximation for all considerations. The most accurate way to measure network speed would be to perform a direct FTP download to your ISP and see the results. The time required to download the test will show you the true network speed.
That should show you how to perform the test on our network.
As far as bandwidth is concerned we monitor the network to maintain the required bandwidth for our customer base. If it appears that usage will affect performance we will upgrade the network accordingly. Here is a link to our CEO blog that will give an idea of our opinion on the matter and how we handle the issue.
Hope this information helps you out!--
DSL Support Specialist