I have misunderstood you with the trust in the dealer, maybe is put my english words not in the right order,I'm a dutch girl, so english is not my main language. I appoligayse for that. Two years ago they sold tosai Momotaro koi in Holland with certificates handmade in Holland

Thats the reason is think that about your question from the certificates!
In november 2005 we bought a nisai Sanke SFF, without a certificate, from the Open Sales, no certificate, she was then 18 months and 58 cm long. She is from the Shobu bloodline. She was in april 2007 68 cm. A very quick grower.
4 weeks ago we bought a nisai Sanke SFF, with certificate, 24 months 61 cm. She is from Akizakura bloodline.
For me that are large nisai, don't you think..
But we have also a smaller nisai showa SFF, see photo from certificate, she was in april 2007, 20 months and was then 50 cm. A huge difference from the above 2 Sanke, don't you think

I think certainly the shoulder is high enough from your Kohaku, she has enough ability, to suprise you in the future, I'm sure about that .
On your certificate is the name from the mother and the month of spawning, and every certificied koi comes from a mother that is sure 80+ cm, all koi from the auction has a certificate. But afcourse you can buy certifcied koi all year round by SFF. But only when they are from a good bloodline, and they are worth a certificate.
gr,
Fukarin