Thank you Nemad for your detailed answer, it is a good step forward for me. I am still learning, trying try everything possible about Knime, and get stuck mostly 
3rd-4th questions are perfectly solved by your answer. I learned something new and very happy right now.
2nd answer is also clear here.
I have something for 4th one, an extension, lets say 4-b
4-b) Final result is showing me the last grid parameters. But I want to see the “optimum parameter set” results. What shall I do?
Get “Best Parameters” from optimization loop end somehow, then again somehow use these in another random forest node as input? How? (1st approach)
Or is there a way to record all iterations, filter the best one among them? (2nd approach)
Also for the 1st question:
1-) I actually trying to use k-fold as a basic precaution against overfitting. I just want to use k-fold with polynomial regression not a bagging model, but I can’t get the final coefficients for full data. It only shows the coefficients of kth iteration when i right click on learner node.
It is a similar problem like 4-b. Isn’t there a way of recording coefficients for each k iteration, then may get average for each coefficient? (It sounds statistically silly, is it possible by the way?)