You may replace the screws with M6 hex bolts, but whatever you use remember not to over tighten them! 22 INCH lbs, is spec... too much and you may warp the covers, split the seal, or prematurely compress the seal; all of which will result in continued leaks.
The ECU requires a signal from the stock oil pressure sender. From what I understand, without it the ECU reacts as if there is a critical fault: pulls timing, adds fuel. Even if you want to swap in an aftermarket oil pressure gauge you really should get the sender working and connected. As btwilson said, it's most likely the connection.
When you installed the head did you place RTV at the corners of the rear timing plate as indicated in the TSRM? If not you're not going to be happy.
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