I think it's really beautiful (& am very jealous that I can neither afford it, nor do I have anywhere to wear it)!
It's not white, it's a rich clotted cream with pastel florals, & is rather Picnic at Hanging Rock (one of my favourite films!).
Yes, there is an implicit tradition that women shouldn't wear a full-on white dress to a wedding - out of respect/to avoid confusion. This has of late been taken to illogical extremes.
The dress is perfect wedding attire. It doesn't look anything like a traditional wedding dress - even less so when you accessorise it with other colours. As for it looking like a nightie - of course it doesn't. It's lingerie-influenced - a trend that is very "in" right now, along with drop waists - but it's been "in" on so many occasions in the last five or six decades (my personal reference period!) that it never really goes out of style, & I consider that dreamy romantic Victorian/1920s aesthetic to be a classic look.
Those who think it's too casual, I disagree with too. From previous MN threads it seems there are now lots of weddings where it's common to wear really revealing tight, short dresses (which, to me, look more like clubbing wear ...!). Fine, if you know that's the acceptable etiquette - but I still think that for the majority of weddings it isn't.
Please wear it to all four weddings, OP - you love it, it's beautiful & perfectly appropriate - & you should get your cost-per-wear down a bit! And with different-coloured jewellery/footwear/bags/jackets, it can look a bit different on each occasion.
(By the way, white wedding dresses didn't start with Queen Victoria - she just happened to be the first Royal bride to wear plain(ish) white rather than elaborate beaded, embroidered, heavy bridal robes. So she merely helped popularise white for the plebs!)