To do this crazy sleeping arrangement

Why are people being horrible to the OP because she doesn't want a floor bed?
She has her reasons. Stop suggesting it. She doesn't want one !
Personally I struggle to get up off a floor bed so I wouldn't be comfortable sitting on a floor bed with a toddler to either read a story or just lay together because when it's time for me to leave I'd be stuck like an upturned ladybird.

P.S. I wouldn’t sleep my kids on the floor unless for high days or holidays.

Yes I’d worry that once she was in your room in the double then you’d have some awful transition later on (with added bonus of her actually being able to get out of bed and the room at that point)
I would go for a single with a bed guard/rail
in her room +/- some sleep training.

If you let her take over your bed it will become her bed. Put a single bed with bed guards both sides, or a wall one side, bed guard the other. It is important not to blur the lines too much and let her claim your space.

YANBU to use whatever sleep arrangement works so that you all get a good nights sleep as long as it’s reasonably safe. Do get a proper bed guard for the double though (even if you have to put pillows against it so she doesnt bump her head and wake up - I have to do this for DS) - I’m not sure pillows alone would be safe longer term as they might slide/slip off.

I’d also suggest trying to get the same brand single bed for her bedroom as you have for the double bed she sleeps better on - that way in the future you can try swopping her back again and she might not notice so much difference (it would be nice to eventually get your bed back!). You could always transfer the same guard rail and prop pillows too - so the only difference would be the smaller sleeping area which she might accept once a bit older.

Why can’t she have a single bed with bed guards?

She’d still have much more room than in a cot.

If she’s that wriggly, there’s always the chance of her kicking pillows off and falling from a double anyway.

If you’re giving her the double, chances are that there will never be a good time to move.

I also hated sleeping in a cot, so was put into a bed at 9months old. This was before cot-beds or side rails so was surrounded by pillows.
I don't see a problem with putting a 15m old in a bed. Can her cot convert to a low cot bed, if not I'd just put her in a bed with side rails.

My eldest was a bit like this. I ended up putting bed slats on the floor within the frame then mattress so he didn’t fall out but it wasn’t so high he’d get hurt climbing over.

To balance things out with the floor beds suggestions:

1. My son had one at 2yo and didn't like it. He slept and enjoyed much more once we raised it (same bed/mattress/etc, just screwed up higher).
2. They can be dangerous for younger children. My daughter loved walking around it when she was 12-15 months old but kept falling (because of the mattress, blankets, soft toys etc), and smashed her heads in the bedguards a couple of times. It hasn't happen once since it's raised off the floor.
3. If OP's DD is sleeping better in the double bed, it's because it's a double bed. So a single, toddler bed, single floor bed won't work anyway. My daughter is the same, she wakes up the minute she hits the side rail.

And why buy a new bed if the existing one can work?!

I also don't think anyone can understand what it's like to have a child who will not sleep if you are in the room, unless you've had one. They will not sleep. There is nothing you can do but get out of the room and let them cry. We got stuck all in one room with our toddler during a holiday, it took 5 hours for him to fall asleep of exhaustion. He went absolutely ballistic! Now that he can speak, he asks us to leave the room to let him sleep.

You are fixating on how strange the set up is. It's not strange at all. Also It's not strange that a toddler needs to be held to sleep either. They are still so small. Making them cry themselves to sleep is so barbaric. They don't learn to sleep or self soothe. They just know you aren't going to come so they are awake but stay quiet. Their stress levels are just as high and it leads to disordered attachment styles in later life.

Why can't she just share the bed with your husband?

Dr Sears (the attachment parenting guide guy) said the best sleeping arrangement for your family is the one where you all get the most sleep. Do it.

You have made up your mind though and it seems pointless that you have started this thread when you won't take anyone's advice. Floor bed is much safer and you could always get a bed frame if it doesn't work out and you end up needing to sleep in there. If the mattress is good quality and comfortable then it won't be any less comfortable than on a bed, unless you struggle to get up from down low but not really a comfort issue, more of a practical one.

Is her room any colder than the others? Mine was waking last winter until we realised how cold her room gets in the night- I think it was down to 10c. Plug in radiator fixed the issue immediately!

I'd worry about potty training with a full height double? Will be she be tall enough next summer to get I and out on her own in the night if she needs a wee?

Floorbed childproof the room and put a safety gate on the door.

Can you buy a montessori bed? it ground level.

I put my youngest in a double bed at about 14 months and it was fine. Didn’t fall out, just used a cot duvet and no pillows. Went straight back to sleeping through after not sleeping in cot.
I also don’t share a room with my DH

keep your stupid faces to yourself

that made me smile 😂

A lot of toddler beds are sort of on the floor but with short sides to stop them falling out.

Google toddler house bed for the style. I'd try one of them.

Otherwise you get the master, snorer in the single with storage in master.

Equally, I am not prone to be silly about cry it out.

It's not silly to not relish the prospect of abandoning your child to cry.

Ah seen your last post. You're too unpleasant to bother with anyway.

Mine went straight onto a double mattress on the floor and were fine. Whatever you need to do 👍

YABU for saying gentler methods won't work with your DD when she's only 15 months old.