Passive infinitive following an adjective
Solution 1:
I am approaching these examples as a native English speaker and a graduate student studying English. I will address each of your examples separately, and then summarize.
The first example is incorrect, as you have it labeled. However, in your correction, you eliminated the use of the passive voice. If you want to keep the passive voice, you need to change your verb to an adverb. For example, "Metal is not easily digested".
The second example is also incorrect. In your correction, you did the same thing that you did in the previous correction and changed from passive voice to active voice. If you want to keep the passive voice, you need to change difficult to an adverb. Technically, "difficultly" is the grammatically correct adverb of "difficult", but it sounds strange and it is hardly used in spoken English. Instead of doing this, you can precede the verb with "too", because the phrase "too difficult" functions as an adverb. Thus, your example could be "Our boss is too difficult to be pleased".
In the last example, the rule that we used in number two makes this example correct, so it doesn't need changed. The phrase "too weak" functions as an adverb, making the sentence "He is too weak to be moved" grammatically correct.
In conclusion, the rule that you mentioned above should actually be "you cannot use passive voice if it is to be preceded by an adjective". You can, however, use passive voice in some cases if it is preceded by an adverb.