Hello Sarieke,
this is actually the main difficulty of greek,that words change meaning depending on the context all the time but I'll try to explain.First of all,in greek we have 3 genders.Masculine,feminine and neutral(sorry if I mispelled any of them ;-) ) Neutral in mostly used for things although that's not always the case for example "the smoke" is "ο καπνός" in Greek with the "ο" and the "-ος" ending showing that this is masculine. A feminine word would be "the noise" = "η φασαρία" and a neutral one "the ashtray" ="το τασάκι". All these also have plural.
If I tried to translate "everyone" I would be referring to either men or women so the translation would be "οι πάντες". In the above sentences if we omit the words "όσα" and "ότι" we would have:
για =for , δεν=not , πρόλαβες=you had the time (προλαβ- is actually "have the time" and by -ες we see that he is talking directly to a person, "you") and ποτέ=never
δεν and ποτέ go together just to show negation,so they only mean "never". So up to now we have "for (blank) you never had the time" .The only word that would fit in english in this case would be "everything" since the final -α in όσα shows we are talking about neutral plural, many things.
The abstract answer to "Πόσα?" (How much?) would be "Τόσα" (As much). "Όσα" is something in between. It would be translated as "as much as (you can handle,you can think of,you can take,there can be etc etc)". So its closest translation is "everything".
Now, the word "ότι" actually means "that" as in "he told me *that* I have to go there" . A variation of this word is "ό,τι" (noticed the comma?) That's actually the word we are talking about here. And "ό,τι" actually has the same meaning with όσα. "as much as (you can handle,you can think of,you can take,there can be etc etc)".
The difference is that όσα is plural so it actually means in this sentence "all of the things that" whereas "ό,τι" is singular and means "every single thing that" with the most simple translation of both being "everything".
Now with "Για δες λοιπόν ". You are correct, δες means see and λοιπόν commonly means so. However the whole sentence is a bit theatrical. Για is used to show emphasis,the colour of the voice and the posture of the person who says this sentence.Imagine a woman pointing and moving her finger at your face and saying "Do you see now you little brat?" The same sentence would be used if you were alone in your room,thinking of something incredibly odd that happened and while nodding in question or despair saying "So see what happened;that's amazing"
The whole sentence could be translated as "Ha,do you see now?" or "Ha,so see" with the word "για" adding that "ha" tone in the sentence, like "I told you so".
I hope I helped and didn't confuse you even more.If you need anything else,don't hesitate to ask.
