For clarity, you're pumping 45gpd of RO, then using the sump's overflow to drain excess, right? It's like a constant water change system with regular RO input?
1) Your overflow box will determine the water line, so you just want it at the lip of the main tank.
if the sump was a 1m x 1m x 1m for simplicity's sake and the tank was 1m x 1m x 2m tall then the max depth for drilling the hole for the pipe from the tank to the sump would be 1m
If you're talking about the siphon break on the return line (from sump to tank), drill it right below the water line of the main. This way if the pump loses power you break the reverse siphon quickly.
2) Yes, and this holds your design back. One option is to boost GH and KH and other minerals daily. Another is to just pump RO into a holding tank and valve the line to the sump. Then you can reconstitute RO in the holding tank and open the valve for a fast water change.
If you want to just use continous RO, you should do it as a top-off for evaporation. You could use a float switch and solenoid if you want to get badass.
3) Normally you just keep saftey room in the sump. For example, you could stick 120L into that ~160L tank, create your siphon breaks, and you're set. Now if you lose power to the pump the reverse siphon and tank drain should only drop a few liters into the sump.
If you're uising an overflow from the sump to the drain, you of course do not need to worry about the saftey room. Instead you should just make sure the sump's drain diameter is wide enough for saftey and redundancy, to ensure the sump never overflows.
4)
Would the flow rate of the pump need to be matched to the flow that could flow from the tank to the sump
No, you want the max flow (drain diameter) from the tank to the sump to be higher than the rate of return from the sump to the tank. This is for redundancy and best practice is to overbuild the drain by ~25%. For example, if you are moving 400GPH from the sump to the main, you'd want a 500GPH overflow, so if you get some blockage at the overflow you're still okay.
It doesn't matter if the drain rate is higher than the pump return rate: water will still drain at the pump's rate. It's not getting into the overflow box any faster. Get it?
The much bigger problem is losing the siphon in your overflow box. Focus here (or just drill the main tank) and you'll do fine.
Other points:
Your UV requires a dwell time and slow flow to be effective. Putting it into the flow pipes is pointless if you want to move a lot of water; instead, consider just sticking it in the sump with a second (small) dedicated pump. It'll work inline if you keep the UV's proper flow rate though.
You don't need really those baffles if you're just putting all the media into the biomedia/sponge tower. Baffles either serve as bubble traps (for SW) or direct flow. Why are you directing flow after the C component?
Good luck.