I had never noticed that koliva for a Slava celebration didn't have a candle, but the explanation on your web-page does a good job of explaining the reason.
It confirmed what I had always understood, namely that the koliva at the Slava is primarily for the departed of the family. (And of course just in any memorial service, the koliva is first and foremost an expression of faith and hope in the coming resurrection to eternal life of those for whom we pray.)
In the different tradition churches I have been to, there has never been any universal custom regarding whether a candle is in the koliva or not.
Don't feel bad about the kolach not turning out right. My wife is an expert baker, and making kolach turn out right is very difficult. She spent a day a couple of months ago with an elderly Serbian lady at our parish, helping her prepare kolach for the lady's family Slava, picking up tips...
Oh, there is a candle -- but it's nbot stuck into the wheet as in the memorial service. The Slava candle is big, usually about 16 inches long or even longer and about 1 inch in diameter. We like to place a little Serbian red-blue-white tricolor at the bottom of it and a little golden cross above it the tricolor.
Below is a picture of a well made kolach. Maybe some will appreciate whay it is difficult.
