Those bowl cover screws can be a beast sometimes.
I used a fresh #2 phillips screw gun bit medium length. Making sure it was straight in the head of the screw, then with a hammer, tapped it a couple of times to get it snug, and then smacked it with 3 hard raps. Then, tapping it again to make sure it was still snug in the screw, slipped a 1/4" drive 1/4" socket over the bit and turned the socket with my 1/4" drive breaker bar. If it doesn't crack loose, try cracking it loose in the other direction (tightening) just enough to get movement (not enough to twist the head off)
On one of the screws I had to repeat the hard smacks a couple of times but it finally cracked loose for me.
Hope that helps.
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