Thank you so much for such a detailed explanation!!
Instead of "Später 'lernte' ich...", I'd say: "Später 'erfuhr' ich..." --> personal preference but it also does sound a little more logicaly
Would it also be okay to say "Später wusste ich"?
Your sentence: "Deswegen ging ich in einen Laden und habe ich <<...>> gesagt.", is almost right. Because you already mentioned the subject "ich" you can't let it away and make: "Deswegen ging ich in einen Laden und habe __ <<...>> gesagt." If you however insist to use the subject again, you just have to put the subject in front of the verb. --> Already mentioned subject = possible to let away / normal construction.
Sorry, I don't get this - you mean the right sentence would be "Deswegen ging ich in einen Laden und ich habe <<...>> gesagt." ??
I hope my questions don't bother you, hahah.