You would get a better response if you search for threads specific to that product. People who are using can better assist and answer your questions directly in there. I will give you a few references to some threads for a few things that you can click on and repost your questions in those threads. The answer to almost all your questions is Yes in some shape or form. 
Could this also unlock the Yale Conexis L1 when we enter the area? Not 100% sure on this one just yet due to the size of the geo-fence area. Wouldn’t like my front door unlocking when we’re far away.
Another Smart Lock SmartApp
Would the Arlo, Nest and Yale app be required on our phones still or could we let the ST app handle everything?
In some cases the native apps still need to be installed. A bigger reason that you would want the apps installed is in the event that the SmartThings hub goes offline or their is an outage in the cloud. You would still be able to operate your devices with these apps as a backup.
If nest works with ST does this offer the same experience as the official app? E.g. Ability to control heating temp, schedules and hot water?
Similar question to above for Arlo, does this show previous recordings as well as things like battery status and live view?
Exterior lighting is currently served by a PIR sensor on the side of the house that activates at dusk and stays on for 8 hours. My plan was to add a Fibaro z Wave relay into this and have ST control the lights to come on at dusk and switch off at a certain time.
Does ST calculate and adjust the dusk time daily throughout the year?
Yes, Sunset and Sunrise is based on your Location defined in SmartThings.