Debugging a PIC16F84A interface with a 4N25 optocoupler
Not trying to be offensive, but are you 100% sure your simulation agrees with the board you're debugging? Is this a production-released board that works, or someone's design concept that hasn't been proven yet?

The _MCLR connection definitely isn't normal-looking to me. Being an active low signal it needs a pull-up (as you've discovered). I also am not totally confident that the voltage divider created with that 1.9k and your 10k pullup is going to work over a large population of parts.

You are also right in that the 4N25 biasing current is way off with the original resistor. The Vishay datasheet only specs CTR at a test condition of 10mA, so your expectation of 20% CTR with 33k is not valid. I generally stick with current that matches the test condition and avoid going higher than that if possible, to avoid premature aging / CTR degradation over time.