If you have someone who is handy, you could get them to build you a cover for that radiator and have it extend over towards the bed with open shelves on the end. Something that looks like this. I'm struggling with importing the pictures. Have a look at that website. There is a picture of a cover that has open shelves at one end - something like that might solve mulitple issues for you. Or, similar to that, it could have a little drawer and door so it would be more like a proper table and then the top of the rad is also a bit of storage for you. This is a Canadian company but the pricing looks quite reasonable IMO. I'm sure you could find someone local to you who could build something similar.

You have a good start, that window placement is tricky! I would put a couple of patterned or darker solid pillows on the bed and a throw at the end. That will help draw the eye from the corner windows and ground the bed. I would abandon the sconces and get a pretty, substantial lamp for your bedside table. Colorful, properly sized rectangular art above your bed will balance with the geometric pattern of the windows and also help draw attention to the bed.

I dont have an app for drawing...but you can close off existing entrance and make new one from living room like a hallway into bedroom.

Do get your teen involved if you haven't already. Provide some options like the daybed vs regular bed option (which I like but may make some teens feel like visitors instead of family). The desk idea may not be as appealing to some teens. Ask your teen if they want a desk or to sit on the daybed with a low table instead. Your teen may not be as into posters as a tv with gaming options. You've probably already involved your teen, but I think it's worth saying. We got a traditional bedroom set for each of our kids and they never used the desks until we moved to a different house with smaller bedrooms. Then we put the desks back to back in the dining room under the center light. Some kids like to be social during homework time. It's hard to know. I certainly didn't. I thought a teen would like to have everything in their room and never come out. Turns out I was wrong and they liked being with each other and with us.