Why can't God speak for himself?
He can, and has. The people did not want that though. Then He sent His Son to speak... some believed/some didn't. But even now, if people say the Holy Spirit speaks to them... most consider those people crazy, without ever even looking at what message they might be sharing.
Why does he allow crackpots with hallucinations, and unscrupulous characters to speak on his behalf?
Why does he allow people to claim they are speaking on his behalf? Should he strike them dead, do you suppose? He sent One to speak Truth. Listen to that One, and you'll know not to listen to the crackpots, unscrupulous characters, false prophets, false teachers, etc.
Why can't God check in briefly to dispel all of the misconceptions and doubts people have of him?
He sent His Son. We just have to listen to Him. And not all the 'theives' claiming that we are to listen to them. Then there would be no misconceptions.
And recall how many people did not want God to speak to them. Its no different today.
Why does he have to wait until it's time to destroy everybody or punish them in hell, before confirming for all the doubters that he exists
He has confirmed it in the past. Didn't change all the people though. He confirmed it though Christ... but even back then when that was happening; people believed what they wanted to believe. And didn't believe what they didn't want to believe.
I don't think Christ returns for the sake of the doubters... but for the sake of the faithful. For those who belong to Him.
That doesn't mean doubters are destroyed or thrown into hell. Only those who would destroy and harm and kill are prevented from doing so (so that He protects those who belong to Him, from those who would destroy them). Those have the "second death". Doubters who have the law (of love) written on their hearts, or who did good to those who belong to Him, are also invited into the Kingdom.
Peace Miz!
Tammy