Couple of years ago I C2C’ed with Inverie as the destination. Don’t believe a word google images show. The road around Loch Arkaig is fine, then you hit the estate road which you secretly hope is going to carry you all the way, but sadly it doesn’t and you fork right over a stile/fence and into a 5-10k hike a bike. Don’t get me wrong, it’s one of the finest hike a bikes’ ever.
On the plus side theres Sourlies Bothy on the sea loch which is fantastic, with a nice descent down into it. Waking up to sea eagles and wild deer in the area is awe inspiring. The final climb over the top is also hike a bike, and, depending on time of year, can also be tick infested. You get a great descent down into Inverie, and can celebrate at The Old Forge with some real ale and seafood.
I would say Spean Bridge to Inverie IS possible in a day, but I’d start early-it’s fairly heavy on the hiking. You’ll be getting a local boat back across to Mallaig in the evening unless you’re staying in Inverie which I would suggest booking in advance. HTH