1. It’s a complement for pardon, which is itself a complement for trouve.

2. Yes.

3. Much like merci, pardon can use the preposition de as well as pour. In both cases, de appears to be more common than pour.

4. Although it’s true that differentiating the two prepositions probably does play a role, a closer collocation suggests that “de” is likely to be used even without a prep. + beneficiary. Of course, you could benefit from carrying out many more such ngrams to determine the exact contexts.

5. No.

6. Yes, though as the other answers point out, wholesale substitution of “pour” for “de” in these cases is a likely anglicism and/or colloquialism, or may even entail a difference in meaning.

As for thematic structures: in English, too, one can ask someone to “Pardon my weakness” (“Pardonnez ma faiblesse”) to “Pardon me for my weakness” (“Pardonnez-moi ma faiblesse”) or to “Pardon me for having been weak” (“Pardonnez-moi d’avoir été faible”) — there are several different argument structures for the verb in both languages. The last is the parallel to the sentence you cited, and the only one in which the the thing one is pardoned for not the direct object and hence requires a preposition. I don’t think the others are relevant.

1. I’m not familiar with “sentence diagrams”, but I’d say it attaches to “pardon”. You can remove “en moi” and “pour moi-même” (one of them or both) and still have a correct, grammatical sentence.

2. Yep.

3. (and 6) First, “to be late” has more than one translation. If you arrive late somewhere, or are behind schedule, it’s “être en retard“. If it’s the time, as in “It’s late, I should get going” it’s “Il est tard“. “Il” is generic, like in “Il est 8 heures“. You can never say “Je suis tard“.

   Second, after pardon (and désolé and merci, they behaves the same), you can use de and pour but for different reasons. It’s a common mistake for English speakers to translate “Sorry for…” by “Pardon pour…” all the time.

   In my opinion, de comes before a verb :

   Pardon de vous avoir dérangé

   And pour comes before a **nominal group* (or a pronoun, it replaces a noun basically):
   _{[see note at the bottom]}

   Pardon pour tout.

   Désolé pour ton vase. Je l’ai cassé en rentrant.

   Other examples with merci :

   Merci de m’avoir ramené.

   Merci pour les fleurs !

4. This makes me realize there are actually more cases. Pardon can mean (at least, I could forget some) two things. It can translate to “sorry”, or to “forgiveness”. In the last case, pour means to whom the forgiveness is directed to someone. The sentence means he can’t forgive himself for not being able to feel love (more or less), as there is no forgiveness in him towards him, I’m not sure of the preposition in English here but I think you get the idea.
   If you use pour to “direct the forgiveness to someone”, the other rules still apply. “Pour tout ce que j’ai fait” could have been in the place of “de n’avoir pu…“

5. I had never seen “Pardonner à quelque chose“, I guess it’s correct but it’s much more common to see “Pardonner qqch à qqn“. I can’t tell if you can say the same with pardon.

6. Again, I’m pretty sure that’s incorrect. You might encounter it somewhere but it’s not good language.

Maybe that was a little confuse so it might need rewriting, please ask me what parts are not clear, I might need an external point of view to make it better.

Also, not that using pardon as “forgiveness” is almost exclusively found it literature, it’s a poetic way to put things, you won’t encounter it in a daily conversation.

[Note] You can also say:

Pardon pour hier soir

I’m not sure in what category hier fits, but it’s basically short for “Ce qu’il s’est passé hier soir” or “Ce que j’ai fait hier soir“, which is a subordinate that acts as a noun. I said “nominal group” but there are a lot of things that can behave just like a noun in a sentence, I didn’t enumerate all of them.

1 & 2. It attaches to pardon:

je ne trouve aujourd’hui nul pardon … de n’avoir su sentir.

The person does not find any "excuse" for not having been able to feel.

3. pardon de exists is probably the most usual form you’ll find:

Je te pardonne de m’avoir fait mal.

Veuillez me pardonner de vous avoir fait attendre.

But please note that pardonner is a actually transitive verb. You can say:

Pardonnez-lui sa faiblesse.

But the following:

Pardonnez-lui de sa faiblesse.

IMHO doesn’t work.

4 & 6. It is not clear whether pardonner pour is correct or a colloquial form. The only example on CNRTL is:

Je vous pardonne pour cette fois.

It might be advisable to avoid it in writing, although of course it’s used in spoken language.

5. pardonner à qqch, as in:

Pardonnez à mon franc parler, je ne sais pas mentir.

Is IMHO now dead, or very rare. I had never heard it before, and it sounds strange. It is noted as literary in CNRTL.