I'm not sure why you don't know what was contributed from employer #1. If you signed up for (for example) $50 per paycheck to be withheld from your pay and sent to the HSA bank, you would know whether you worked for 9 or 12 weeks, and you would have seen the withholding on your pay stub. Likewise you should be able to look at your bank account online or paper statements and see what deposits are credited to you, independent of form 5498.
Assuming the $450 is correct, it would be best to not file your return until your employer issues a corrected W-2. Get an extension if needed. If they refuse to issue a corrected W-2, then I would file by creating a substitute form W-2. This can be triggered after you enter your W-2, there is page of check boxes for special circumstances and one should be "I received an incorrect W-2 or did not receive a W-2." Enter the W-2 using the figures you believe are correct (in this case, $450 in box 12 with code W, and everything else the same). For the substitute W-2 you will probably be asked to type a brief explanation. Make sure you keep your records including your employer's refusal to make a correction.
Then of course, you only need to remove the excess $450.
The path of least resistance would be to enter the W-2 as-is and remove another $150. The cost to you is that you will pay income tax on the $150 and you will have a little less money in your HSA for future medical expenses. But, if you enter the W-2 as-is and don't remove the (likely false) excess, you will probably get an automatic letter from the IRS assessing the tax anyway, and then you have to mail in an explanation to the IRS.
