Canisetr check valve to slow the flow, harmful?

You don't want to limit the flow of a canister too much. It will burn up the motor head.

I was reading a great article about this the other day but unfortunately I cannot remember where it was.

The flow rate is fine as is for free floating algae. The slower flow rate is required for killing parasites.

I have 2 Fluval canisters which support reducing the flow by no more than 1/2. Not certain it's okay for all canisters. I wouldn't recommend it. Equipment is designed and tested to perform under certain conditions and any change you make compromises its performance. If there's a question about causing damage to the filter by restricting the flow, then don't.

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I have a number of Eheim Ecco canisters where I just use Seachem DeNitrate which requires slow water flow to work correctly. I have flow ball valves in the output lines which reduce the output flow by more than 75% and they have been running non stop for many years with NO problems. I have always heard than you do not want to reduce the input flow to much but it is fine to reduce the output flow. That being said I run sponge pre-filters on all my normal Pro II/III Eheim canisters (which run wide open) and when they get dirty the flow is greatly reduced for days sometimes before I notice it. The sponge filters are cleaned off and the flow returns to normal volume and this has been going on for years as well. Eheim Ecco canisters are cheaper and have NO burn out protection so must have some water going through them at all times. The more expensive Eheim Pro II/III models have burnout protection and it is not a worry for low flow volumes.

as jcahow said above, pumps can't stand having their INTAKE throttled back, but throttling the output is usually OK.

for your situation, however, why not just put a bypass (with a valve) around the UV light to keep the total filter output steady, while limiting the flow rate through the UV light however you need?

It is unlikely that filter is going to have room for such a DIY project. I have one. The UV filter is inside the canister, and the media baskets have an opening (that doesn't have much spare room at all) in it, and UV is connected to the lid.