Glad you found a fix! That area you cleared and the gutter at the top of the A pillar can direct water into the cab when plugged.

From my experience, the main leak was my cracked intake plenum. That's the plastic piece at the bottom of the windshield that spans the width of the truck. The cracks in mine were allowing water to drip into the heater intake.
I think to add a deflector or cover to the heater air intake, you'll need to remove the intake plenum. Be careful with that! It needs to twist a little to get in and out of place. That plastic can get very brittle.
I could reach up and touch that intake area without removing the intake plenum, but it never felt like I could reach it well enough to attach a deflector. It could be worth a try. It's a narrow rectangular opening at the top of the passenger firewall. I don't have my rover anymore to post any photos to help direct you. I think you could fashion a deflector from a plastic soda bottle or some similar material. It wouldn't take much to provide a little coverage.
My fix was the new intake plenum, and while it was removed I put silicone around all the seams of the heater intake opening. I didn't add a deflector while I was in there, but that would've been the time to do it. I didn't find a need to add the deflector after the new part and sealant.
Hope that helps you.

 

I made a heater intake deflector for mine. I don’t think you could do it without removing the plenum. I don’t remember that being a big job.

 

I feel like it was removing the wipers mostly and then unclipping/unhooking the plenum from its place. I did that about 4 years ago so hard to remember exactly. I feel like I found a write up somewhere about it. Maybe it's a step in replacing the windshield?

 

Yes, I replaced the intake plenum/center air intake/windshield moulding this weekend (great how to video at Atlantic British!) since it was cracked in the middle and missing both end pieces that divert water to the drain area. After cleaning out the drain holes and the entire area in front of the air intake vent, I had no water last night after a pretty good rain! I can see how, when the moulding is removed, that you could re-seal the fresh air intake fitting below the windshield. If the leak returns, that will be my next step. Removing the moulding should be easy since it is new and pretty flexible but I will leave well enough alone at this point. I did check my A pillar gutter area and didn't see any cracks so I assume that I wasn't getting any water down the A pillar. Thank you everyone for your input and help!!

 

I just cleaned mine out. passenger side was completely blocked with black crap up inside the nipple.