I haven't read evey response here yet but I just want to point out a few things about females that should help the OP in this thread and maybe a few other lost souls.
These are a given with every female, I haven't met one exception to this rule yet.
1. Females love male attention.
2. Females love male attention from both someone they're dating AND guys they are not dating.
3. Most females are completely satisfied with the level of male attention they get while in a relationship and do not go seeking it elsewhere.
4. When a female is no longer in a relationship with a male, this burning desire for male attention claws at their very being.
5. Females in need, and with no immediate males around for attention will always, always always reach out to males from their past, friend or ex in hopes to get a compliment or two.
6. Do not fall for the guise, this is simply females scratching an itch they have, very similar to you having to beat off regularly. It just has to be done.
Lastly, the most important rule when it comes to females is this.
7. If you have to ask, the answer is ALWAYS, No.
In the OPs situation, lets look at the facts. He's been crushing on her for 20 years... 20 years. He has spilled the beans to this girl, so she is well aware of his feelings. However, through 2 decades she has managed to keep things as a friend relationship. It seems the OP has always just 'missed' his opportunity with her between other boyfriends. This is incorrect, girls don't mess this kind of thing up. OP has always been her friend and will always be her friend.
In the latest meet and greet, the OP finds out she is maybe single again. What does she do? She calls him for lunch, and possibly dinner with someone else there?
1. Red flag, she simply phoned you to hang out of the blue? This is basic male attention grabbing at its finest. Nothing wrong it per say, but ... its right there, clear as day.
2. She probably already exhausted her other male outlets.
3. She KNOWS she can get that sweet male attention from the OP and keep her distance at the same time.
4. How? Because she's been doing it for 20 years.
OP, my advice is to save everyone but mainly yourself a heap of time, and worry. Call her after you read this message, call her and ask her flat out if she is single and if she was wanting to hang out because she is interested. This shouldn't be hard to do at all, I mean you've known her for 20 years ... do it. She sounds like a like minded person, but you need to cut through the BS sometimes to get an answer, thus is the dance men and women play with trying to relate and understand one another.
Wondering this or that, asking strangers on the internet is going to do nothing. My opinion is that she is just using the ruse of being a female to be coy and just mysteryous enough to drag you along just long enough for her to latch onto another male. Nothing wrong with this, I am no knocking women, but this is a pattern I havent' seen an exception for.
Girls are like monkeys and men are the branches they swing on. They will NEVER let go of one branch until they have a firm grip on the next. Never forget this.