прочитаю is just the future of читать.

Сейчас + future tense means I'm going to do something as in right now, without delay, etc.

Сейчас читаю твое сообщение и отвечу is outright incorrect as the sequence of tenses is not there. Читаю is present and continuous in this context, отвечу is future. You can still make it right though, something like:

Сейчас читаю твое сообщение. Потом отвечу.