I must admit the storyline does seem rather rushed. With Ross Kemp leaving EE soon there doesn't seem to be enough time for this story to develop further.
This storyline regarding the reveal of Grant's son Mark would only have worked properly if:
Ross Kemp would have agreed to a minimum 4-6 month stint
Had Grant had returned for a longer stint, this would have allowed the storyline to develop before and beyond the big reveal, including Grant getting used to the idea of having a son and him interacting with both Michelle and Mark and eventually getting to know his son properly on-screen.
Bringing back Michelle Fowler either with Susan Tully returning or a new actress to play her.
Michelle really needs to return for this storyline. It seems really odd that Michelle, who has been keeping this secret for so long, for her to not want to defend Mark or at least try to keep the secret safe by returning to the square. I know the ex producer admittedly failed to persuade Susan Tully to return as Michelle when he was in charge, but they should have re-cast her, it's been over 20 years since Michelle was last seen so surely this would have worked if they had got the right actress for the job.
As the story was rushed to fit in with Ross Kemp's 3 week stint, we'll probably see Grant storm off on Friday never to be seen again and Mark to return to Florida.
The dream scenario would probably be for Grant to invite Mark back to Portugal with him and for him to accept but surely this won't happen this quickly after Mark knowing the truth. Time will tell