I HAVE SOLD SOME STOCKS THAT I HAVE NO INFO ABOUT COST BASIS. CAN I REPORT COST BASIS ZERO?

You would have to find, estimate or guess your cost basis by searching past values for these stocks.

You can of course report a 0 cost basis, but then you would be inflating your capital gains and pay more capital gain taxes.

ONE OF THE STOCKS HAS BEEN CREATED ON 2024 FROM THE MERGE OF 4 OTHER COMPANIES ON APRIL 2024, I FOUND HISTORICAL DATA FROM YAHOO FINANCE. CAN I USE THIS PRICE AS THE BASIS? 

IF THE COST BASIS I REPORT IS HIGHER THAN IRS CALCULATIONS, WHAT WOULD BE THE CONSEQUENSES?

Yes, you can use that value as your Cost Basis.  Keep documentation showing the amount you used with your tax files. 

Here's more info on How to Find a Stock Cost Basis.