I think the first option would be easier than a HDMI splitter. Just tell them to select input 1 when they watch TV. No problem, TV gets sound and pic from box. When you want to watch it in HT, select input 2 and turn on the AVR. A universal remote with macro like a Harmony 890 would also make it easy. have a family button that selects Input 1 and a HT button that turns on the AVR and selects Input 2, etc.

Panny 3000 PJ, 118" Carada, Denon 3300, PS3, Axiom QS8, PSB 5T, B&W sub, levitating speaker wire