Can I charge DELTA/RIVER and use them to power devices at the same time ?

Yes, you can. 
You can charge and discharge the power station at the same time. This applies to all EcoFlow power stations. 

Can RIVER/DELTA function as UPS?

All EcoFlow power stations can be an emergency power source (EPS), which will have the same result as UPS for most electronics. Here's the difference, when your power station is plugged into the wall, anything plugged into it gets power from the grid, not the power station's battery. If power from the grid stops, the power station automatically switches to its battery supply mode within 30 milliseconds. This is a basic UPS function, and it does not support 0ms switching. So do not connect the power station to any device that requires 0ms UPS, such as data servers and workstations.
Also please make sure that the power station is charged to 100% when you want to use it as EPS. 

https://ecoflowtechhelp.zendesk.com/