First death - and others to follow?

[F3l1ss]

I don't think the ornament did any harm, I've never heard of them causing deaths in that way unless it was coated in something. Did you wash it first?

Heaters are rarely accurate to what they say, you often have to figure out what to set them to against an in tank thermometer. So if it's going higher than you want set the heater lower than what it says and monitor the temp.

I think it would be wise to follow Martinspuddle when he said:
As you already have your 24 litre, if you move water, decor, filter media and stock into your new aquarium there shouldn't be any issues. Do a water change 50% water change the following day and do daily testing for the first couple of weeks.
Just don't be tempted to add anymore stock for a while till the bacteria has had chance to settle.

While you'll have to dose larger quantities than you would for 24l, I think the benefits of more room for the fish and more water to dilute any pollutants would be worth it.