I'm not sure if you want to call her ahjumma. It might make it sound like she's old. Eon-nee means older sister, more suitable.

Both have pretty much the same meaning. 고맙습니다 is more like "Thanks." Like, someone does something kind (someone holds the door open for you, gives you a gift.) Usually, you'll hear 고마워(요) from friends or family. 감사함니다 is the universal/formal way, "Thank you." which is why you most often hear it in business or in public.

Noodles

Yes, similar to Spanish. There are 6 forms. I'll use "essen" (to eat) as an example. ich esse - I eat du isst - you eat (familiar) er/sie/es/man isst - he/she/it/one eats wir essen - we eat ihr esst - you eat (plural) Sie essen - you eat (formal) Sie can also be used for plural and formal. Like, if you are talking to a group of professors. The conjugations are generally consistent but like most other languages, "to be" and "to have" are irregular.

Solamente hablar. Algunos dicen sin ㅎ porque es más rápido y fácil.

그래? as a question means "really?"

You could use 그리고. Like 조쉬 기타를 쳤어. 그리고 집에 가서. I think 그럼 is just a response. Like "Are you leaving? Then I'll see you later." 그럼 갈게.