First the setup with using just the Scarlett as a single audio device (which is what you have pictured for the Reaper Preferences/Audio/Device page).
You have control of sample rate and block size disabled in Reaper. (By unticking those boxes next to the controls.) First choice is normally to control these parameters from Reaper (ie your host DAW). Disabling this and controlling from a 3rd party app should usually only be done if the connected interface demands it (ie refuses to talk to Reaper).
Did you try this from Reaper first?
You're not currently selecting the aggregate device from Reaper (per the screen shots). But looking at the clocking setup...
Aggregating a hardware interface with the computer's built-in can have issues. There's no word clock connection for the computer's built-in interface. That means you need the clock connection over the data connection (firewire/USB). Ticking resampling for both devices in the aggregate is not going to work no matter what. I would try clocking from the built-in interface (since it has no hardware clock input and thus that is flat out not an option) and make the Scarlett chase that (tick the resample box).
I wouldn't necessarily want to record anything critical with the Scarlett inputs while it chases the built-in interfaces clock over the firewire connection. Use that for the convenience when both devices are required only.
These would be my first choice settings:
Using the Scarlett by itself:
Tick the boxes next to sample rate and block size in Reaper and enter the intended values. (You can open Audio MIDI Setup and verify that the Scarlett is getting the message.)
Using the Scarlett + the computer built-in audio interface:
Make the built-in audio clock master in Audio MIDI Setup.
Tick the resample box next to the Scarlett.
Select the aggregate device in Reaper device menu.
Again, tick the boxes to enable control of sample rate and block size with Reaper.
Did you try those already?
Is it correct that you are doing any monitoring of live inputs with the built-in cuemix mixer in the Scarlett? You're not actually trying to run low latency live sound with Reaper? If so, try a block size of 512 or 1024 samples (a large value to save processing power for the mix).
More on making an aggregate device with built-in interfaces + external interfaces:
This can be dicey because we're sending clock sync over a computer data connection which isn't exactly recommended. I blathered on about some observations doing this in this thread recently:
https://forum.cockos.com/showthread.php?t=208737