We are building a quadcopter and a fixed-wing for a local competition but found that no stock around is available of any Pixhawk (we have no time to order abroad). So we went for APM but looks like it is no longer supported by the ArduPilot. How bad will it perform in an amateur flight if we use APM with the latest supported release (looks like too much, since there is ~2000 lines of release notes since the last supported one)?
Also, I wonder whether we can make ArduPlane/Copter work with Arduino or something. Porting some core functionality only will suffice (flying the classical quadcopter and fixed-wing frames autonomously with a GPS, accelerometer, compass and gyroscope; plus servo control) hoping that no hardware constraints are to be faced.
The APM will work, but you’ll miss out on a lot of support and features/fixes.
An Arduino is only good as a companion computer, it cant effectively fly a quadcopter with native Arduino code. It can only do one thing at a time, only one loop. A flight controller with a real time operating system has multiple loops running to perform various functions at appropriate rates.
This is wrong. Look at the Arducopter Changelog from 3.2.1 (last supported for APM) and current Stable. That will tell you all you need to know. 7 yrs of development, hundreds of advancements and an 8 bit board to 32 bit. Arducopter Changelog
The thing is that ArduPilot is no longer about just APM and Pixhawk. There are lots of other hardware options, where the flight controllers listed in the Closed Firmware section are generally cheaper and more available. The cheaper ones may be somewhat less reliable (e.g. have less redundancy or less precise sensors), but for the non-professional use this is more than OK.
APM is theoretically possible to fly, but it is really hard to do. You have to go with a really outdated version of the firmware and will have lots of collisions with the current documentation. The best answer to any serious question will be checking out the source code and reading it. This steampunk experience is not something a new user wants to have.