No, you can still have air trapped in the rear caliper/wheel cylinder. Removing the line could introduce air to the caliper or wheel cylinder.
Unfortunately, this means you might end up replacing your rear calipers / wheel cylinders also. It has happened to me a number of times. Be sure to spray it with some good penetrating oil and use a 6 point socket.
Your procedure for bleeding is fine, except for the banjo bolt
You can bleed a line at the banjo bolt but you can't bleed a caliper.
You can get your stuck nipple off if you break the head by using a bolt extractor. If you use a bolt extractor and have to drill it out in any way you will NEED to disassemble the caliper to ensure it does not have any metal ...
Referencing the photo it appears the retaining clip is still in place. At the 12 o'clock position you will notice a square shaped tag. Lifting the tab away from the bracket pry the clip in the direction of the small hole that the tab sat in. Again referencing the photo this would be going up. With the amount of corrosion visible I would try to wire brush ...
It's more likely that you (Cooked) the sender unit for the oil gauge. Making it read improperly. Heat can damage alot of things on a motor. Also most later model cars have a (Low) pressure safety threshold. (ie) if the oil sensor is cooked and does not register enough oil pressure at turn over' The car will never start. This is easy to see on a functioning ...
Pontiac is a GM brand of vehicle, so yes, GM parts will fit your car. Pontiac parts are not hard to find in the States ... pretty much any parts shop will have them or can get them. Not going to talk about prices, as that would make this off-topic.
If the issue suddenly appeared after your repair, check the following:
Did you use the correct model of spark plugs?
Are the spark plugs correctly gapped for your engine?
Did you use the correct ignition coil for your engine?
Did you reconnect the correct spark plug to the correct ignition coil output?