Yes, you can release the Rowlet whenever you like. It will not affect your chances of getting a Shiny Rowlet.
I don't think you've understood how probability works. There is no such thing as a Pokemon "counting toward the odds". Each Pokemon you encounter is not ticking down on some timer that will spit out a Shiny when it hits zero. Shiny hunting is random (at least, as random as a computer can make it).
All you are doing is repeatedly trialling the 6/4096 chance you have of getting a Shiny on any particular encounter. None of your encounters are guaranteed to give you a Shiny, no matter how many encounters you do; you just have to keep trying and see if you get lucky.
Are you ever guaranteed to roll a six on a die? No, but if you keep rolling it, it'll "probably" happen eventually -- same principle applies.
There is a rigorous process called binomial probability that will tell you how likely you are to have at least one Shiny after x number of encounters, but even that makes no guarantees. You could do 100,000 encounters, and there'd still be a tiny fraction of a chance that you don't get a Shiny. The good thing is that's exceedingly unlikely.
