I want to speak to you.
 I am looking forward to seeing you.
 I am interested to learn English.
No, the to-infinitival is not a myth, nor is "to" always a preposition.
However, I can see where Mitchell is coming from. You see, the subordinator "to" that marks infinitival clauses derives historically from the preposition "to" (notice the ...
To is not always a preposition.
See Merriam-Webster here. Examples:
The children ran to and fro. (adverb)
He finally came to. (adverb)
His bride-to-be is very charming indeed. (adjective)
The problem underlying your concerns is that grammarians do not always agree on whether to is a particle or a preposition in to + infinitive.
I follow ...
I heard him sing.
This implies that his singing is finished. "Sing" is a bare infinitive.
I heard him singing.
This denotes that his singing is ongoing, and has not come to an end. "Singing" is functioning as an adjective, qualifying the pronoun "him".
"going to" and "not going to" can be used in multiple ways.
They can be used to make predictions, as in
It's going to rain tonight.
It's not going to rain tonight.
But if the speaker is a person in authority, this prediction is often used to express an imperative. For instance, a parent might say to their child
You're going to be home by 9pm.
All I did today was + verb
In this type of structure, both to-infinitive and bare infitive are grammatically possible. However, the use of bare infinitive is more commom and idiomatic.
All I did today was hang around the house and watch TV. (More idiomatic)
All I did today was to hang around the house and (to) watch TV. (Less idiomatic)
Can the structure “sb doing” replace a clause?
It is grammatically correct to change this to
I suggest that your question is not correct, once again the question is about grammar whilst disregarding meaning or use. "Such" is a word used to emphasis things. So what is the important thing in this sentence? The Blessing or that he has come home safe? I would ...
They're all fine in terms of grammar. If you want to express that you would like to write a book before you turn 40, option 1 seems to me the way to go. My sense is that "would like" is softer than "want," and maybe less intentional.