Problems with the basic plot

This is an entertaining movie, well-made and historical and all that stuff. But when I look at the basic plot of the movie, it is fairly weak. The Oz stuff is a dream (in the movie version), but still Dorothy did not know this, and we are not to know this until the end; and so it should follow some logic, understanding that the movie occurs in a setting of Munchkins and witches and flying monkeys and a talking lion etc.
Dorothy wants to go home upon arriving in Oz. She told this to Glinda (good witch), and Glinda told her to follow the yellow road to get to the Wizard in order to get home. But, Glinda could have simply told Dorothy that she could go home right now (and never have to deal with the Wicked Witch and not have to travel the yellow road etc.) by clicking her heels. Instead, Dorothy was nearly killed by the Wicked Witch because of this journey. The only explanation given of this at the end (when Dorothy basically asks this question of Glinda) is that Dorothy would not have believed Glinda, but this makes no sense. Why would Dorothy have not believed a good witch at that time? Dorothy certainly believed Glinda in following the yellow road to go to a wizard, which is no more incredible that clicking one's heels. Dorothy then says some jibberish about home ("If I ever go looking for my heart's desire again, I won't look any further than my own back yard. Because if it isn't there, I never really lost it to begin with.") But Dorothy left home not to seek fortune or adventure (in the movie); she left only to save her dog, and after meeting with the Professor she wanted to go back home. So Dorothy always understood the value of home. Dorothy did not really grow or learn anything along the trip. The others changed during the trip, but not her. So the trip seems to have served no purpose for her.
And if Dorothy could kill the wicked witch with water, why did Glinda not give her that valuable tip? (It's hard to believe that Glinda would not have known this. And Glinda has no problem intervening in such things as she took the shoes from the dead witch and gave them to Dorothy, and she intervened during the poppy scene by making it snow.) Dorothy was saved only by a quite random event, i.e. the witch deciding to burn scarecrow, Dorothy trying to save him, the presence of a bucket of water, and the water accidentally hitting the witch. It's not as if Dorothy and the others defeated the witch through wit or perseverance etc.
And as for the Wizard, he sent Dorothy (a child from his world) on a life-threatening mission, when he apparently had no intention of helping them (as after they got the broom he told them to come back later), when he could have simply sent her back home in his balloon from the beginning, and he comes across as this kind old man. He only helped them when Toto fortuitously pulled back the curtain, and he decided to come clean and help them (and to go home himself).