Where are the in-ceiling speakers located in your den? Will they work as rear surrounds in relation to your Arc?

Right above the sofa.
Yep, they should work a rear surrounds. 

The Arc/Amp/in-ceiling speakers would be a good option. If you don’t like the way the in-ceiling speakers sound, you could purchase a second Sonos One SL and use the pair as surrounds instead.

If you have the budget, add another Amp to power your outdoor speakers.

That’s what I was thinking as well! When I originally purchased the amp, I thought (incorrectly) that I would able to connect the in-ceiling and the outdoor speakers to the same amp, and be able to have different volumes for each pair of speakers. Recently determined that it’s impossible to do w/ a single amp.  I don’t want the outdoors speakers to play every time I am watching TV inside, so I’ll probably just leave the outdoor speaker disconnected for now, or purchase a used, inexpensive amp outside of the Sonos eco system for the outdoors speakers. 

Another option would be to get a used Connect:Amp (Gen 2) for your outdoor speakers. They are selling for around $300-$400 on eBay. It’s not as powerful as the newer Amp, but it should be enough for a pair of outdoor speakers. And you would be able to play the same music outside as in your den which is always great to have especially for entertaining.

That’s a great idea! Also may use an older Pioneer that I have for the outdoor speakers for now. 
 

Will the Connect:Amp connecting the outdoor speakers also be able to play whatever is playing on the TV inside? 

Yes, you can group the den room or zone to the  outdoor zone so the same audio will play through all of your speakers. Be sure the Connect:Amp is the Gen 2 so you can use it with the S2 app.

Thanks! That helps a bunch!

Last question — Is there any way I can integrate my sole Sonos One SL in another room to play whatever is on the surround sound? 

You can group the two rooms, with two caveats. 1] it won’t play the surround sound, it will play all 5.1 channels at the same time (it’s a mono speaker), and 2] there will be a 75ms delay between the Room that is the Arc and any grouped room, but only when using the TV input. When streaming music, they’ll be in sync. 

@Airgetlam I’ve seen you mention the 75ms delay before. Is the delay very noticeable? When I group my One in the kitchen with my Arc setup in the living room, the audio sounds pretty in sync to me.

It’s a matter of your brain, honestly, as well as the distance between the speakers. To my brain, it’s painful, somewhat like listening to a singer in a stadium where tryouts can hear the singer and the stadium speakers at the same time. Slightly out of phase, but not certain death. On the other hand, there are folks who don’t have the speakers sensitivity as me, for which either they don’t care, or the ‘other’ speakers are far enough away to have the sound travel over distance not make a difference.

I can’t play my stereo Sonos Ones in my open plan kitchen at the same time as my Arc. But that is  my layout, my ears, my brain. 

I prefer to warn, and let others choose to be worried about it or not, but at least if there’s been notification, they can’t come back and complain ;)