Yeah, I used to share my drop from the cab with my neighbor for 3 months whilst virgin sorted out a re-pull for them, they don't get any access to your equipment its simply just sharing the frequencies sent down the cable.
And as far as your Wireless being "hacked" goes, it don't seem correct as when you was doing the speed test's it would of pulled a lot more bandwidth than you shared with us due to the load balancing..
And..
The only issue I know of with wireless security on the SH2 is that when it is starting up after a restart the networks broadcast for some seconds un-protected and don't require a WPA passkey to be entered, In that time the "hacker" can login to the SH2's GUI with the default admin password and get the WPA passkey so that once the network secures its self they can login "legitimately"
That's why I always recommend changing all WiFi names and passkeys and also the GUI passwords too as most ISP readily configured routers have these issues due to them using calculators to make the security details and later on you can just enter the SSID online and get the passkey to login.... I know as i have done this in the past with Sky, O2 and BT routers and that's not to mention the wifi security gap that can be taken advantage of too.
MLM