You can connect as many TVs as you like to the HughesNet modem, whether via WiFi or LAN cable, though there will be a limit with LAN cable connections if you don't have a switch. This does not mean, however, that you aren't restricted by available bandwidth, as that bandwidth will be shared between all active devices, though not necessarily evenly.
As well, you can use any WiFi extender you like. You don't have to use HughesNet's WiFi Booster. Plenty of people use WiFi extenders with HughesNet, including the inexpensive plug in models on Amazon. How well they work, though, will depend on the quality of the extender. The HughesNet WiFi Booster, however, can ONLY be used with HughesNet's HT2000W modem.
If you're having issues with streaming, the first thing to do would be so ensure your service is operating properly. To do so, go to this modem page. If both boxes at the top of the page are green, it's fully operational. The next step would then be to test your speed, which you can do following these instructions, and then post the results page URL so that the test results can be viewed.
A couple of things you can do to try to make the streaming work a bit better is both reducing the resolution of the stream, either at the source or via your device if you're using an app, and turning off, or snoozing, the Video Data Saver.
If you feel that you were misled by the representative during your sales call, you can request a sales call review in the "MyAccount and Billing" section. If you do request a sales call review, make sure to specifically reference the discrepancy. With this said, sales calls are only available for review for about three or four months after the call.