[HPsauce]

A few points about the scuttle and aircon condenser (orange) drains:

The orange drains, in theory, only ever get totally clean water so shouldn't get debris. In practice the tray is open to cabin air so some dust/debris gets in. BUT if the scuttle drains overflow muck from there gets in somehow making it much worse. The tray holds several pints!

Cutting the tips off the orange drains "cross", so they're a mm or so open seems to help reduce blockages.
In an emergency the passenger side trim panel is much easier to get off so clear that drain first. The drivers side needs the knee panel removing first which is a recipe for broken fixings.

When refitting (I dip in boiling water!) the two smaller collars go either side of the transmission tunnel metalwork, push through with fingertips until it feels fully seated.
There is thick insulation inside the cabin that the larger collar seals against. The cross end obviously points down out of sight.

I find that ensuring the scuttle cover is precisely fitted to the base of the windscreen and cleared out regularly reduces the frequency of blockages.
If it doesn't fit snugly debris, leaves etc. just wash down the windscreen and straight into the drains.
You can peer into the tray (under the HVAC air intake unit, from nearside is easier) with a torch to check for water build-up.

I use a flexible plastic pipe and "suck and blow" down it to agitate the debris, after first syphoning off any deep water then adding hot water and detergent.
In my case the blockage seems to usually be in the tray, once that's softened by heat, detergent and agitation it "whooshes" out through the pipes.
I think once warm the exits there are large enough and far enough down that the flow just goes straight through. No need to trim them.
(Though they are almost totally inaccessible anyway unless the engine is out!)
Preventive maintenance of very hot water and detergent flushed through regularly I heartily recommend. I've got an indoor watering can with a very long spout that holds a couple of litres and does the job well.