grammar - Dependent clause after pronoun

This question arose from why sentence #1 is correct and why sentence #2 is incorrect -

I pity those who lost their money in gambling.

I pity them who lost their money in gambling.

I have asked the question in ELL forum, as well as in Linguistics forum.

But the answers their made me more confused.

RULE 1

Jlawler's comment contains the direct answer to the question. Definite personal pronouns (I/me, you, he/him, she/her, it, we/us,
they/them) cannot take a restrictive modifier. In other words, they
cannot take a dependent that narrows the set of entities that they
denote. This trait of personal pronouns underlies their use as test
words for constituent structure. For example:

 (a)  The man with the hat knows the woman with the scarf.

 (b)  He knows her. 


 (c) *He with the hat knows her with the scarf.

Sentence (a) is the starting sentence. Sentence (b) shows proform
substitution; the personal pronouns he and her have been
substituted in for the noun phrases the man with the hat and the
woman with the scarf. Based on the acceptability of sentence (b), one
concludes that both the man with the hat and the woman with the
scarf are constituents. Definite pronouns such as he and her (and
them) take the place of constituents, in this case of complete noun phrases.

The unacceptability of sentence (c) reveals that the strings the man
and the woman in (a) are not constituents. In other words, the
definite personal pronouns he and her cannot take dependents
(=modifiers), since they necessarily replace an entire noun phrase.
This fact explains why them who lost their money in the question is
bad English. The relative clause who lost money is a postdependent
(=postmodifier), and as such it cannot modify them (because them
as a definite personal pronoun cannot be modified).

The plural demonstrative pronouns (these and those) behave
differently. They can take postdepndents (=postmodifiers, i.e. a
modifier that follows them), e.g

 (d)  These with hats know those with scarves. 

This is simply a trait of the plural demonstrative pronouns (these
and those) -- there is no good explanation why plural demonstrative
pronouns behave differently than definite personal pronouns; they

simply do. Note that the plural demonstrative pronouns also behave
differently than the singular demonstrative pronouns in this regard,
e.g.

 (e)  *This with a hat knows that with a scarf.

Singular demonstrative pronouns (this and that) are behaving like
the definite personal pronouns; they cannot take dependents.

The combination plural demonstrative pronoun + restrictive relative
clause can actually be viewed as a particular construction in
English and related languages. That is, it is a combination that
occurs relatively frequently and has therefore been lexicalized.
German has a very similar construction, e.g.

 (f) Diejenigen mit einem Hut kennen diejenigen mit einem Schal. 
     those      with  a   hat  know     those   with  a   scarf.

By acknowledging that one has a particular construction, one is in a
sense admitting that there is no real grammatical "explanation" for
the phenomenon. It simply exists.

Finally, note that there are certain apparent exceptions to the
principles mentioned above. There are uses of personal pronouns that
actually allow modification, e.g.

 (g) He who studies a lot gets a good grade. 

In this example, the personal pronoun he is not referring directly
to a specific entity, which means it is not definite; it is, rather,
being used as an indefinite pronoun; it means 'the one, anyone', e.g.
Anyone who studies a lot gets a good grade.

MY CONFUSION

Reading this answer make the following sentence consider wrong -

It is she who stood second in class.

So another person came up with another rule -

RULE 2

Nominative personal pronouns can be modified by relative clauseas just
like demonstrative pronouns; it's the objective personal pronouns that
can't. He who, she who, they who, you who are all grammatical, if
archaic. Him who, her who, them who, however, aren't.

MY CONFUSION

Now this rule create a conflict with the rule 1 I quoted first. In the first rule it says - He with the hat knows her with the scarf - sentence is wrong, but if we consider the second rule then this particular sentence should be correct.

Another problem with the second rule is that it makes the following sentence incorrect -

The action was performed by her who is the secretary of XYZ company.

So another rule came in picture -

RULE 3

"Them," combined with the "who," has to be used with a preposition
like "to," "from," or "with."

"I pity them," by itself, is a grammatically correct sentence, but
when you connect the dependent clause with "who," it is no longer
correct.

Now I am really confused. Can anyone here please help?