Thank you for your interest in Stellar and for your excellent question!
I do not work for or with the Stellar organization, but wanted to take a minute to follow-up on your question because I support their objective, I support Jeb's work, and I honestly believe that Bitcoin and Stellar have great futures.
Question: Why does Stellar need to have 80% of the supply locked up?
Answer: I'll refer you to the official description here. As for my individual answer and interpretation, I'll say that out of the total mined Lumens, Stellar has committed to only truly 'locking up' 5% or or 5 billion Lumens for the purposes of operating their business which is understandable. They started off with a value of $0 USD and only by their, and our as a community, hard work will they be worth anything and so really its of no consequence at the start. 5 billion of nothing is still nothing.
Theoretically those Lumens are worth about $5 billion. But is there anyone out there right now that would pay that much for any or all of them right now? I don't think so, so again they are currently virtually 'worthless'.
Stellar, as a non-profit organization, can and does accept tax-deductible donations to help them fund their operations as well.
I don't have time right now, but in the newsletters and blog posts from Stellar, you'll find information about what their plans are for the remaining Lumens they hold. For example, Stripe receive a whole bunch of them for their use in building a payment integration system, which Stripe subsequently has donated a majority of them to others for a similar purpose.
Many have gone to build challenges awards to help fund those, like me, who are building systems that integrate with Stellar. Millions of them are given away to those winners every few months or so.
I trust that they will continue to donate, give-away, and strategically sell the remaining Lumens as it is NOT in their best interest to hold on to them in the long run.
You also asked: Is the Stellar core centrally controlled by Stellar and will it always be centrally controlled? Meaning Stellar can update the software and change the source code at anytime?
Stellar is a truly distributed and open network, unlike Ripple. Ripple, while 'open-source', is a closed system that only they choose who can and cannot join the network.
Stellar, on the other hand, is completely open to anyone who wants to spin up a core and/or horizon server and federate a trust connection to any other server. That means if I spin up a server, and you spin up a server, you can choose to trust my server.. or not. Nobody prevents you either way.
There is a great short interactive book that shows you how it works. I can't find it right now, so if somebody knows it please share it here.
Even if the entire Stellar organization went away, the network would continue on without them, just like Bitcoin goes on without Satoshi's influence.
It is true that Stellar can update the software anytime they want, but that's why it's open-source. We can all see exactly what changed and build it ourselves. You could run your own network on your own custom version if you wanted, nobody can stop you.
Your last question was: Why isn't Stellar more upfront about how centrally controlled it is on their website?
They are up-front about it. If you follow the blog, newsletters, and forums you'll see they don't 'hide' it. But, I'll be honest, it is one of the downsides of Stellar vs Bitcoin. Bitcoin is based on zero trust and Stellar requires some trust, but they both have pros and cons regarding this.
Hopefully I've answered your questions thoroughly and I hope that you will stick with the network and help all of us build it. Sometimes the worst critics of something can turn around and become the greatest proponents. Have a great day!