That reminds me of a episode in Friends where Ross drove to the airport and met Emily at the gate. He told her he loves her and wishes to spend more time together and she said thank you before going on the plane. Then later, Emily calls him from US since she was looking for him and Ross was in England. She said she love him and he said "thank you." XDDD
I sometimes said thank you to my boyfriend. Though I don't think he felt content when I said that... Even though we have said I love you to each other for a lot of times for years, sometimes I just blank out and say thank you when I'm not focused in the conversation. '___' I want I love you to be said with feelings, not just to be replied back to.
If you're not ready to say it yet then yes it is perfectly okay. Say "I love you" when you are ready. Don't rush just because your partner is there already enjoy the journey at your own pace. If it's the right person they'll wait for you to catch up.
The context of the question does not limit the conversation to the first "I love you". It's not bad to be grateful to be loved. While in my country Its easier to say "Te amo/i love you" to a lot of people in your lives, its always good to add "gracias, yo tambien" (thank you, me too)
It is/could be lol, depends on context and the relationship/understanding between the two. I'm sure it can be fine but a 'I love you' back might work better or a sweet smile. It could come off as selfish or not caring about your partner though, and people can misunderstand easily. Best to take the time to really get know them and communicate. All the time people say I love you with just a look, communication and understanding is key in a healthy relationship.
Getting really upset if someone says "thank you" seems a little... selfish to me. As though confessing you love the person is more about your own need for their reciprocation, as opposed to how you feel about them... If you really, truly love someone, I think it's important to understand that they may need time to say it back without feeling pressured or uncomfortable.
Love is deaf. If they show their feelings towards you and you FEEL loved enough to say it to them in the first place, that says something right? It'll mean more if you give them the chance to say it when they are ready and just accept the "thank you" for now.
I would say, if you can't say "I love you" back to them for some reason, sometimes saying nothing is better than "thank you", because "thank you" sounds like a slap in the face if they were hoping you loved them back. But I've learned that saying "no, you don't" is the worst response al la my ex boyfriend, lol.