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But He promised that the way to the Kingdom of God would be a secret, a mystery except to His disciples—and they would be very few.
Say what?

I think you need to show the rest of the quote. When Jesus was on earth relatively few were able to understand the parables he used:

But He promised that the way to the Kingdom of God would be a secret, a mystery except to His disciples—and they would be very few.
He told His disciples: "It has been given to you to know the mysteries of the kingdom of heaven, but to them it has not been given . . . Therefore I speak to them in parables, because seeing they do not see, and hearing they do not hear, nor do they understand" (Matthew 13:11, 13).

Only by having the spirit can you know exactly HOW to get to the kingdom of heaven:

1Co 2:14 But the natural man receiveth not the things of the Spirit of God: for they are foolishness unto him: neither can he know them, because they are spiritually discerned

As for your example, yes, keeping the commandments CAN bring eternal life. But we can't keep the commandments perfectly, physically or spiritually. However, when the spirit of Christ dwells in us, can stop doing our own will and let the spirit of Christ live in and through us. This spirit WILL keep the commandments of God insofar as we let it.

I read Matt 13:11 and replace 'of' with 'about'. The Lord is speaking about how Heaven is organized, angels, uses, correspondences, etc. He spoke in parables so those internal things would not be profaned.

The Lord's speech is Divine -- it had meaning to all that believe in Him. Then as now.

The whole of the NT deals with salvation, repentance, how to love the Lord and the neighbor to lead a good life according to one's religion. That's what leads to Heaven.

You quoted Corinthians. Look at the beginning of the next chapter...

3:1 And I, brethren, could not speak to you as to spiritual people but as to carnal, as to babes in Christ. 2 I fed you with milk and not with solid food; for until now you were not able to receive it, and even now you are still not able; 3 for you are still carnal. For where there are envy, strife, and divisions among you, are you not carnal and behaving like mere men? 4 For when one says, "I am of Paul," and another, "I am of Apollos," are you not carnal?

Paul is challanging those to raise their mind to the spiritual, which really anyone can do who is sincere about his faith.