I'm sure it's been explored before, but that's not how it works. If China started dumping their Treasuries lead to significant domestic inflation, which they definitely don't want.
Also, minor point, Japan holds more Treasuries than China.
That's not how US debt works. It pays back on a schedule. China could stop buying new US debt, and will continue to receive payments on US debt it owns, but China can't call up the US and demand full repayment immediately.
They could sell the debt that they currently hold, sure. The response would likely be a short term rise in interest rates followed by the Fed simply intervening and purchasing Treasurys.
And we could sanction the PBoC and freeze the accounts into which we repay the debt they hold. The China-owns-our-debt line ignores the realities of sovereign debt.