In the area in which im in, i.e UK/Ire i’d say the majority would pronounce it Mad- UH- LYN as in [name_u]Lynn[/name_u]/Lynne although i must say i actually prefer the Mad- UH- LINE pronunciation akin to how its pronounced in the children’s film [name_f]Madeline[/name_f] 1998. It just sounds much more polished and refined and classy in my opinion!
My sister’s middle name is [name_f]Madeline[/name_f], pronounced -lynn. I’ve never heard [name_f]Madeline[/name_f] pronounced -line (I live in the US) so I think you’re fine!
If it’s spelled -line, it’ll be pronounced line until corrected. There are different spellings for a reason. Maybe spell it the [name_u]French[/name_u] way [name_f]Madeleine[/name_f] so you get the -lyn/luhn sound.
I’m from the UK and I’ve never heard anyone pronounce [name_f]Madeline[/name_f] as mad-uh-LINE in real life, only on the internet. [name_f]Every[/name_f] [name_f]Madeline[/name_f] I know is mad-uh-LYNN. I don’t think you’d be regularly correcting people.
If I had a [name_f]Madeline[/name_f], I’d go for -LINE so the rhymes in Madeline would work. But all the Madelines I have known IRL have pronounced it “lyn” like you want.