Ooooo this is complicated. Let's break it down into parts.
Q1: in a rhubarb of a pickle of a jam =? Is it slang words? I guess it refers to in trouble. Yes it is slang...more like an expression. If you say you are in a jam...you are in trouble...if you describe something as being a pickle...it is complicated. So if you are in a pickle of a jam...it is a complicated problem...and I think the added rhubarb in there just to make it silly. You can have Rhubarb jam...to put on toast...so it is just playing with the words.
Q2: a corker of a case =? I don't know the direct meaning of corker but it means that the case is an interesting or special one. Another expression: It's a doosie! It just means that people will probably find it interesting or strange.
Q3: I don't understand the connotation of words which I highlighted in red. Seems a man drove up onto a traffic island and hit a decorative rowboat full of geraniums. Now they're trying it as a maritime offense.
So, a man drove onto a traffic island (this is the center part of the road that you don't drive on and it usually has trees, grass or flowers on it). So the man drove onto the traffic island and crashed into or hit a decorative rowboat (a rowboat that is not real...but only for decoration) which was being used as a planter for flowers (geraniums). Because the driver hit a boat (even though it is just decorative), they police or court are charging him (trying him) with a maritime offense. (Because he had an accident with a boat not a car.)
Does that make more sense? I hope so.
Have a great night!