Steinbeck created this sense of a cycle because that is what life is for us. Every book has a purpose to comment on real life issues that we might not only enjoy the text but be challenged to think about how this issues play out in our own lives.
Obviously a sharp theme in this book is sacrifice. George has sacrificed much of his potential for pleasure so that he can take care of Lennie. By this point in the story, he makes the ultimate sacrifice by giving Lennie a pain-free death and positioning himself to be caught by Curley as if he was doing Lennie a favor so Curley could not torture him.
The imagery of the location reflects complete and total serenity and perfection, something we hope for in an after-life. Hopefully George just sent Lennie there.
I’ll address the first part of your question. I’m not sure if your book is the same as mine, but look in your novel to see if the section with the heron eating the snake are on these pages. This image from nature foreshadows the unexpected demise that Lennie will endure. The theme this image reflects is survival.
George essentially puts his closest friend out of misery because he rationalizes it as the most humane action he can take and it is really the only way he can continue. George cannot allow Lennie to be put through a slow death.
In the book Of Mice and Men the story opens with the two men, George and Lennie, heading towards the river. It then focuses on them at the river. The author describes the setting in detail. It is as if he is also placing the memory of the river scene in the reader's mind as much as George is placing it in Lennie's mind.
George instructs Lennie to remember the different things about the place so that he can use it as a future safety zone. The author foreshadows that Lennie may have to return to the place.
On pages 99-100 Lennie has gotten into trouble. He now has had to recall the information George had told him in the beginning. By returning the same setting, the reader knows that Lennie has followed George's directions. The reader is also aware that Lennie knows he is in trouble and he goes to the safety place as foreshadowed in the opening chapter of the book.