I would rotate Mangan and Rose and Marriott in a 442 system.
I would play Mangan as Advanced Forward. I've discovered that in the lower leagues, determination makes a HUGE difference. His determination will make up for his low finishing skill.
I would use Rose as the Withdrawn Forward with instructions to run with the ball and train him to round the keeper. I'd first improve his Finishing until it's 10 then train up his Composure. This might take 2 years, btw.
I would rotate Marriott in to keep Rose and Mangan rested. I'd train up his dribbling first so he can slot in at either role.
I'd loan out the rest if you ever get in any financial trouble.
Best of luck!