Absolutely! Maintaining great relations with ex-employers is super critical!

By being a resource and a great contact it gives you power in your next interview that you are still valued at your last place of employment!

Good for you!

Absolutely. It appears they are seeking you out. Hopefully, a great offer is in the offing. Even if it is just social, you can check the climate there. And they are probably buying, so you get a good lunch.

I think that depends on why you left that company. If you left on good terms and have positive feelings about them, it can't hurt to see what they are thinking. On the other hand, if you wouldn't go back there for any amount of money...you're wasting your time (and theirs')...

Yes if the lunch is job related and it all depends on what the meeting is all about but a definite no if you feel like that now that he or she is no longer your boss for there may be some type of emotional/feeling from where they may have fantasized about you. But only you have to make that decision based upon the possible scenarios that could possibly happen

Hell no. I wouldn't even offer them a glass of water in the Mojave Desert.

Don’t expect anything. Just play your game. Be positive.

Of course you should! Forging and cultivating relationships is the cornerstone of a successful professional career life!

By maintaining manner and good education it will be possible.

I think it is important to keep in your network. If you didn't burn the bridges when you left the company, it has always job opportunities available.

Certainly! If you parted on good terms, maintaining good relationships and networking with those with whom you can have mutually beneficial relations can be great. Furthermore, you may find that they are a good source of information, referrals, etc. in the future. I have done this in the past and actually was glad that I did.