Assuming you’re in the USA …
These guys https://www.pbgc.gov/ are in charge of defunct pensions. They were created by Congress in 1974 in response to a bunch of high profile pension failures that left retirees and soon-to-be retirees flat broke.
Check with them about the details of your #1 pension. Not all companies are covered and the details are arcane.
As to #2: That’s a big question.
They can at almost any time freeze the pension. Which means that upon your eventual retirement you’d be entitled to the benefits you’ve earned as of today, but would not continue to accrue any increasing benefits during your remaining years of employment. I personally have a pension that was frozen in 2012 and although I’ve worked there ever since, my benefit isn’t growing; it’s frozen as of that fateful date.
If your management can arrange to declare bankruptcy, perhaps as part of an arranged merger, they can also terminate the pension plan.
If terminated and the plan was underfunded versus its needs, then PBGC would take over. And would pay out some fraction of the value you thought you’d earned. PBGC stretches the available money to cover all the people owed by two means:
Limit anyone entitled to a large pension to only collecting a small one instead.
Divide the total money available by the total owed (after the adjustment in step 1) and pay everybody their “fair” share. Which might be 80 cents on the dollar or might be 15.
All the above is a massive oversimplification.
But the short version is your pension is only safe if your employer isn’t merged, sold, hostilely taken over, bankrupted, or run by crooks. And that has to stay true until the day you retire and take your money as cash, not as a monthly payment.