Hi my dd age 2.9 is also in a todler bed and uses the full size single duvet on it but te ,wrong way rond, it therefore tucks under oneside of the bed and is less likely to get kicked off, also higher tog than cot duvet.
she also has a ready bed (older age group one) which she has used at my parents for approx year, always seems warm enough in it, and yes it zips up so covers don't fall off.
Not sure what you meant about putting the ready bed on the normal bed? They contain the blow up mattress so she has always used it like that . They are narrower than a standard mattress