--邀请午餐这种礼遇就表明很感兴趣
Yes. If they are interested enough they can create a position for you. Another possibility is that they want to know the "inside info" about their competitors.
--我该如何表现才得体呢？有哪些需要注意的呢？
Google "interview meal", "interview lunch", you'll see a bunch of suggetions for what you should wear, what you should order, how you should eat and talk.
--穿面试的非常正式的白衬衫西装套装 Business Suit 还是稍微不那么正式但也是平日上班装那种？
Depends on their dress code in company, you'd better a little bit (just a little bit) overdress.
--但对如何表现有毫无头绪，该多讲还是少说？该尽量表现推销自己还是含蓄儿、不问不答？
Moderate.
