20.2 French Etiquette, questions for the usage of le/en/on

Bonjour tout le monde, j'ai queleques questions.
I'm extremely confused in lesson 20.2 with "le / en" and the usage of "on".
I've tried to put all the pronouns back. Please check it for me. Et voici-moi quelques conseils. 

à ce qu'on dit? (so they say?) 
Why "on" is equal to "they", instead of "so we say" in this sentence?

..., on m'a dit (..., I was told. )
Couldn't she use "je m'a dit"?

Ne t'en fais pas. = Ne te fais pas inquiète.
Is there any condition that someone may say "Tu ne te le fais pas"?

Tu dois les poser près de ton assitte. = Tu dois poser tes mains près de ton assitte.

Si on t'en offre un, de cadeau, ouvre-le tout de suite = Si on te offre un cadeau, ouvre de cadeau tout de suite. 
(If someone offers you one, as a gift, open it straightaway) 
Why the first one is "en", and the second one is "le"? Does it depend on verb?

et si ce sont des friandises, distribues-en à la ronde en remerciant. = Distribues des friandises. 
(and if they are candies, pass them around thanking people.) 
And why here is distribues-"en"?

I really can't explain myself the endless nightmare of le/en/on. Please help!!!
Merci beaucoup.

«On» is used as a generalized "unspecified" person or source, or as means to create passive construction, so it is also used to replace "one", "they", and "it". It's not just a "colloquial we" substitute.

«Je m'a dit» doesn't work because je is the subject but a is the auxiliary verb for il/elle/on. If you say «Il/Elle m'a dit», it means that this person is identifiable. It's not just a notional "It's said that..."

To put the emphasis on you and not the "sayer" of the thing, you can use the construction «J'ai entendu parler...» (I've heard...)

The expression/verb phrase is «s’en faire» (to worry) and not just faire (to do, make), so «en» is integral to the expression and it can't be substituted. «Tu ne te le fais pas» would mean "You don't do it" (with the "it" being previously identified as a masculine noun like «jardinage» (gardening) or something like that).

On the cadeau question, the first translates to "If you are offered one of them". It's partitive, so you use «en». It isn't because of the verb; en refers to de cadeaux. The second is a continuation of the previous idea. You only received one gift--un cadeau--so you're only opening one (ouvre-le).

If you received two, the construction would be: Si (l')on t'en offre deux, ouvre-les tout de suite. (If you are offered two of them, open them right away.)

On the friandises question, the en refers to des friandises. As with the cadeau question, this is because of the partitive.

Thank you very much, it's very clear, and I try to put them into conclusion:

a)
« on m'a dit » =  Someone told me...
« Il / Elle m'a dit »  = He/She told me...
« J'ai entendu parler » = I've heard that...

b)
 « s’en faire » =  « se faire du souci »  so it's..
Je ne m'en fais pas.
Tu ne t'en fais pas.
Il ne s'en fait pas.
Nous ne nous faisons pas du souci.
Vous ne vous en faites pas.
Ils ne s'en font pas.

c)
A: Je voudrais faire du jardinage dans le jardin de mon voisin.
B: Non, tu ne te le fais pas! C'est malpoli.

d)
When the partitive (du, de la, de l', des) noun is turned into pronoun, in french it's replaced with en.

Are they correct?
 

Hmm...it looks good except for the tricky construction in (c) because of the imperative. To think of it as a command  « Ne te le fais pas ... », it would need something (another verb) in the ellipses. Otherwise, it would need a partitive application again (Don't do any of it) because gardening isn't countable.

How about:
Quant au jardinage, je ne le fais pas depuis un mois parce qu'il pleut souvent.

Aha torusan, how about this conversation?

A: Je voudrais  commencer à faire du jardinage.
B: Non, ce n'est pas une bonne idée. Nous n'en faisons pas depuis un mois parce qu'il pleut souvent. 

Yeah, that works! We haven't done any of it. :)

D'accord, merci