I think curtains similar to the previous owner would work if you keep the colour or background colour close to the wall colour. It's hard to find pictures of a bay with a lowered ceiling without shutters etc but you could also have a pelmet and curtains on the main wall outside the bay or have curtains on the main wall maybe with romans on the window. If you take the curtains higher than the bay ceiling it will make the windows appear visually taller. There are lots of rooms with the coffee table much further away however as you actually want to use the tables for tea etc you may want to keep the 45cm gap but have wider coffee tables. My table is 44cm away (to be precise!) and I couldn't reach much past that comfortably TBH. I would try it! I've just attached 2 pics showing the sofas overlapping the chimney breast and not to see what you think. The chairs in the bay will be fine if the main seating area is zoned with a rug. I've seen your other posts re David Linley, his silhouette chair is rather lovely!

You can't get more sturdy than this Digs daybed as its made of Steel. :-) however if you prefer a wood or a softer look, then my Client has just gone for one from the Futon Company, you can find them online if there isn't a store where you are. They went for the Quad 3 seater futon day bed. www.futoncompany.co.uk

Search for porch swing daybed. You can find some on Etsy for example. https://www.etsy.com/nl/listing/501894121/free-shipping-the-avalon-bed-swing-twin?ga_order=most_relevant&ga_search_type=all&ga_view_type=gallery&ga_search_query=porch swing&ref=sc_gallery_2&plkey=fb6327fffeef6c54f1f204d7e56f04c24a603e05:501894121

The advantage of the hemnes day bed is it can convert to a double so could last until adulthood in theory. Would you/he consider a midsleeper? There are loads on Wayfair (with good reviews). Or if you have space how about getting him a double bed? https://www.notonthehighstreet.com/cuckooland/product/four-poster-king-bed-with-storage-and-shelves-in-white?istCompanyId=aa76f5e6-d733-4e56-8409-574cea196cc9&istItemId=xmxqqlqmlp&istBid=t&gclid=EAIaIQobChMIsNL0_MvS2AIVBbftCh3chQwaEAkYDCABEgKgj_D_BwE&gclsrc=aw.ds&dclid=CLHk74LM0tgCFcaxUQodgwICeQ http://www.ikea.com/gb/en/products/beds/double-king-size-beds/brimnes-bed-frame-w-storage-and-headboard-white-lur%C3%B6y-spr-59124616/