I haven't had much luck trying to destroy assets to affect stock prices either. But here's what's been working for me so far. It's not a guaranteed quick-billion but over time you'll get there.

Invest in Debonaire!

The stock price for Debonaire consistently fluctuates between around 4.8 and 7.4 (at least from what I've seen in my ~60 hours of story mode gameplay). Since the price is so low, that tiny change is actually a significant percentage. If you invest everything in the stock when you see it anywhere below 6 or so and then sell it back any time it goes over 7, you're making a decent percentage (up to 54%!) back on your investment at a relatively frequent interval.

If you are actually trying to reach $1,000,000,000 then this method is going to take a long time. There's probably something better out there. But at least it's something reliable to do until someone finds that better thing.

As for precisely how long, given your starting balance of 26 million and assuming an average ROI of 10% (YMMV) and ZERO extra spending, you'll have to invest and sell everything 39 times to reach a billion dollars. But only 19 times to afford the golf course.

You can't quick save while on a mission, sadly. You'll have to wait for the mission to end (you get a little summary screen and are generally asked to press A/X to continue) before you can use the phone to quick save.

I've also been in a state before where I couldn't use the quick save feature even though I was pretty sure I'd finished any active missions. In that case, sometimes killing myself reset things. Other times that didn't work and I had to load my last autosave instead.