Can I get away with hiking shoes instead of boots?

Only if you are an experienced hiker with strong ankles and are carrying a lightweight pack. For the average trekker carrying a 30lb+ backpack, the ankle support of a boot is worth the extra weight.

Do I need fully stiff mountaineering boots?

No. You need a trekking boot with some flex. Mountaineering boots are too stiff and will cause blisters on the long, flat sections of the trail.

Choosing the Right Hiking Boots for Patagonia and Torres del Paine

Prioritize a mid-to-high cut waterproof boot with a stiff sole to handle Patagonia's rocky, unpredictable, and muddy terrain. Break them in for at least 30 miles of walking on varied surfaces before your trip to prevent blisters.

Select for Terrain, Not Just Weight. Patagonia trails are rugged with sharp loose rock (scree) and deep mud. Avoid lightweight trail runners unless you are an ultralight expert. Look for a boot with a Vibram or similar high-traction outsole and a protective rubber rand around the toe.
Ensure Waterproof Integrity. Gore-Tex or similar membranes are essential. Patagonia is famous for 'four seasons in one day.' If your boots aren't waterproof, you will have cold, wet feet by noon on the W-Trek.
Get the Right Fit for Downhill. Size your boots half a size larger than your daily shoes. When walking downhill, your toes should not touch the front of the boot. Wear the specific medium-weight wool socks you plan to use on the trail when trying them on.
Break Them In. Never wear brand-new boots for a multi-day trek. Spend 30 miles hiking on uneven ground to soften the leather and identify hot spots. Apply moleskin or tape to these spots immediately if you feel friction.

Can I get away with hiking shoes instead of boots?: Only if you are an experienced hiker with strong ankles and are carrying a lightweight pack. For the average trekker carrying a 30lb+ backpack, the ankle support of a boot is worth the extra weight.
Do I need fully stiff mountaineering boots?: No. You need a trekking boot with some flex. Mountaineering boots are too stiff and will cause blisters on the long, flat sections of the trail.