I kept them at 30 degrees for 16 hours and 20 overnight.

Why? At what time of year do alder seedlings naturally germinate and
grow in the uk? What temperature is it normally at that time of year?

You've been trying this for a long time, so you must have found this
page for example: https://treegrowing.tcv.org.uk/grow/tree-recipes/alder

From the picture it looks like they have no trouble germinating
thousands outside. I imagine they would give you advice if you asked
them.

Peter