You are right for the fact that the info on available balance for exchange is needed, i don't know why kraken don't give it.
Probably in the simple interface of the standard site, they forgot...
I think you can also try to contact the support, and ask for this feasture, i don't know if they listen to users sugestions but giving a try, why not ?
I'm an experienced trader, so for me the pro version is what i usually use in all the exchange i use, so for me is not a problem, but i understand that not all people like to use a so rich inteface UI.
Cheers
I've contacted the live support from Kraken finally. And their answer has been to invite me to use pro.kraken.com instead, as you did or another Kraken website/UI :
https://trade.kraken.com ... or to deduct manually the amount of my orders from my balance
They said they will forward my query about the legacy interface though.
I don't like very much the trade.kraken interface so I decided to switch for the pro.kraken one. It reminds me the one of Bitfinex a little bit, and I start to appreciate it.