In the past, Nice to meet you was considered the proper response to an introduction or short conversation with someone new. But like everything else, language changes and "Nice meeting you" is perfectly acceptable.
Since great is not on the list, there is no reason it can't be followed by a gerund, so, again, you're safe there.
I have not been able to find the rule you gave. However I have found a list of adjectives that require the "to + infinitive". They are:
adjectives expressing emotion, e.g. angry, disappointed, glad, sad, happy, anxious, pleased, surprised, proud, unhappy, confused, befuddled...
adjectives of ability or willingness, e.g. able, unable, due, eager, keen, likely, unlikely, ready, prepared, unwilling, willing...
adjectives used to express opinions, e.g. to give opinions: difficult, easy, possible, impossible, hard, right, wrong, kind, nice, clever, silly, foolish...
adjectives referring to difficulty, e.g. difficult, easy, possible, impossible, hard
when using the preposition "of" with other adjectives:
It’s kind of you to help.
It would be silly of him to spend all his money.
This was all I could find. Sorry.
Source: British Council
Q1. How to distinguish between past participles used as adjectives and past participles that have a passive meaning?
Past participles that have a passive meaning have an explicit or implicit agent.
- She is admired by everyone who knows her (explicit).
- Your help is appreciated (implicit, by me).
- The price of petrol has been reduced (implicit, by the oil company).
Such past participles are typically not modified by very, but by very much or an alternative adverb:
- She is greatly admired. (?She is very admired.)
- Your help is very much appreciated. (?Your help is very appreciated.)
- The price of petrol has been significantly reduced. (?The price of petrol has been very reduced.)
Past participles used as adjectives very often describe mental or emotional states, and therefore have a person or animal as their subject. There is no explicit agent, and often not even an implicit one. Such past participles are typically modified by very, not by very much.
- I'm very bored. (?I'm very much bored.)
- John's been very depressed for several days. (?John's been very much depressed for several days.)
- She looked very disappointed. (?She looked very much disappointed.)
Q2. Which adjectives to use with very and which adjectives to use with very much.
As noted in the section above, past participle adjectives that describe mental states are generally modified by very, not very much. Most other adjectives are also modified by very alone. However, there are some common exceptions. For example, adjectives that describe extreme qualities are not usually modified by either very or very much (?very enormous, ?very much wonderful). There is another group of adjectives that needs a different modifier than very (fast asleep, wide awake, far apart, well known, etc.)
Q3. "This looks very like what we had at our shooting party in November." Is this sentence is really correct?
A short answer: yes, This nGram shows is very much like to be more common currently than is very like, but before 1940 the reverse was the case.
In answer to your supplemental question (How to determine which form to use?), my recommendation would be to invest in a good grammar of English usage* to learn about general patterns, and run an nGram as above on specific instances (or do a simple Google search on the two phrases: for example "is very like" gets 411,000 hits, while "is very much like" gets 36 million - so it is clear which is the preferred form).
*The two books I consulted in preparing this answer were Swan's Practical English Usage and Collins Cobuild English Usage.
Best Answer
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.