Je veux remercier Marie maintenant.

In French, when you use "vouloir" (to want) followed by another verb, that second verb stays in the infinitive without any preposition.
• You say Je veux remercier (I want to thank), Je veux manger (I want to eat), and so on.
• There’s no need for a marker like "to" or "de" here.

Proper names don’t need articles in French unless you’re referring to them in a special way.
• Marie is just the person’s name, so it doesn’t require la or any other article.
• If you were talking about “the Marie we met last week,” that would be a different context, but here it’s just Marie.

Yes, French adverbs like "maintenant" can often move around for emphasis.
• You could say Maintenant, je veux remercier Marie or Je veux remercier Marie maintenant.
• Both are correct, though putting it at the end might sound a little more natural in casual speech.

Yes, you can.
• Je voudrais is just a more polite or softer way of expressing I want.
• Je veux is more direct, often used in everyday speech, while Je voudrais is common when you’re trying to be polite or make a request.