Good fundamental questions here describing the usual doubts people reach in electronics.
why do we still use conventional current to model circuits
Keep this in mind: Negative charges moving one way correspond to positive charges moving the other way.
The current arrow represents positive charge flow. Always.
You are in the beginning of electronics studies and only encounter rather simple circuits at the moment. Therefore you only see electronic current (i.e. electrons being charge-carriers). But there are many other possible charge-carriers:
- negative electrons in metallic conductors (metal wires, usual circuitry),
- positive "holes" in semiconductors (PV solar panels, thermocouples, transistors),
- positive and negative ions in a mix in conductive fluids or electrolytes (batteries, fuel cells, the human body),
All such charge-carriers can appear in an electric circuit. A mix of positive charge moving one way and negative the other way in different parts. Luckily, a negative charge moving one way always corresponds to a positive moving to other way - an electron leaving a spot leaves that spot more positive than it was before, corresponding to it gaining a net positive charge. So, due to this equivalency, someone has back in time decided to simplify all talk about current and chose that whenever we talk about current, we mean the direction a positive charge would move.
This same consensus was made for several other topics as well, such as electric field direction, magnetic field direction etc.
- [...] If we open the switch and make this circuit an open circuit the bulb should still light up since electrons will flow from the negative terminal to the bulb and there is no switch to stop the electrons from reaching the bulb.
This is true - for a very short while. When the battery (voltage source) is turned on, electrons will move from negative terminal and as far away as they can. Through the bulb, yes.
But soon the first electron reaches the end and can move no further. More and more arrive and accumulate there. As you know, like charges repel each other, so the more that accumulate there, the more do they prevent further electrons from arriving. This slows down the current until it stops.
This is the reason no steady current can flow in an open circuit. Current can flow momentarily, but not at any constant steady rate. And the momentary flow in the beginning might not cause enough power on the light bulb filament to heat it up to glow.
- Now assume that there's a resistor in place of that switch and an LED in place of that bulb [..]. I can suppose that the resistor is there to prevent high voltage from reaching the LED so that it would not fry but again assuming that we make a circuit exactly like explained above the LED would still fry up because the current would reach the LED first and then the resistor.
Again, a LED and a light bulb will both "fry" (the electrode/filament will melt) at a too high temperature. Although large current happens momentarily, and although large power may happen momentarily as a result of that, there is not enough energy transferred to heat up the filaments to the limiting temperature. Or at least not for long enough time for the filament to actually melt.
When light bulbs do burn out, though, it usually does happen when turning it on of this exact reason with momentarily large currents. You may have experienced turning on a lamp just to see it light up in a big flash and blowing right away.
So why place a resistor up near the positive terminal, it should have been placed near the negative terminal so that the resistor dim the current before it reaches the LED.
There is no difference between the positive and negative "side" of the electric circuit. While electrons will be moving away from the negative terminal and follow the description I made above, they will in a similar way move towards the positive terminal at the other side. Remember that electrons are already present in the metallic wire before hand; they are not "sent out" from the battery only. So the exact same explanation and effect can be used there. There is no "safe area" in such a circuit.