I don’t think that should be an issue. If it’s not really an arb, then you shouldn’t worry too much. Also, you are just getting started so bookies should cut you some slack.
That should be fine. And as for the mistakes, don’t worry it happens!
I have many times backed a lay’ed different horses, even added an extra 0 to a lay stake!
Btw, I just started a Facebook group for everyone to talk all things matched betting! Feel free to join by clicking here!