I have open source and open hardware sensor that would give you a working starting point: it is internet connected and transmits its temperature, humidity, and battery voltage every two minutes an will last for 3-5 years on 2xAA batteries. It is based on the M12 6LoWPAN module.
I'll try my best to grab touch on all of your questions:
Regarding band tradeoff:
433MHz, 915MHz, 2.4GHz
Range vs. antenna size is the clear tradeoff here. Free-space path loss is a function of wavelength so lower frequencies travel much farther for the same attenuation. BUT, in order to capitalize on this you'll also need a suitable antenna which also scales with wavelength. The 2.4Ghz antenna on the M12 takes about 2 sq. cm of PCB area.
A second factor is licensing. 2.4GHz can have unlicensed stations worldwide. 915MHz is only unlicensed in US (it's a GSM band everywhere else). I'm not sure the restrictions on 433MHz.
Data rate is also effected by frequency choice according to Shannon–Hartley theorem; you can cram more data into a higher frequency band. This isn't always used for more final data rate though. 802.15.4, for instance, has 4 bits of redundancy for every real bit seen at the data layer. The 32 symbols are pseudo-orthogonal so you have to corrupt several low level bits to cause an error. This allows 802.15.4 to operate under the noise floor (research suggests at -5dB SNR) and makes it relatively robust to interference.
Now on to the next hard topic,
low-power radio operation:
Compared to household battery sources (e.g. AA alkalines), even the "low-power" SoCs such as the mc13224v aren't very low power. The transmitters are around 30mA at 2-3.5V and the receivers are 25mA or so. Without turning the radio off and putting the CPU to sleep, this load will drain 2 AAs in a few days. The high power consumption of the receiver is often surprising to people and probably the biggest pain in developing low power radio systems. The implication is that to run for years, you can almost never transmit or listen.
The goal to get "year long" operation from 2xAA alkalines is to get the average current of the system to be < 50uA. Doing so puts you at years and up against the secondary effects from the batteries such as self-discharge and the 7 year self life for household batteries.
The best way to get under <50uA average is if your transceiver doesn't need to receive. If this is true, then you can "chirp" the data as quickly as possible and put the system into a low power mode (say approx 10uA) for most of the time. The TH12, for instance, transmits for about 10ms, but there is other overhead in the system regarding processing time and setup times for the sensor involved. The details can be worked out with a current probe and spreadsheet:
From that type of analysis you can work out what the run-life is going to be (assuming you have an accurate discharge curve for your battery).
If you do need to receive data on the low power side (e.g. to make a sleepy router in a mesh network) then the current state-of-the-art focuses on time division techniques. Some tightly synchronize the network, such as 802.15.4 beacons, and others use a "loose" system such as ContikiMAC (which can be easier to implement esp. if your hardware doesn't have a stable timebase).
Regardless, my experience shows that these methods are around 400uA average which puts you in the "months to maybe a year" run-time with 2xAAs.
Collisions:
My advice: don't worry about them for now. In other words do "aloha" (your option #1) where if you have data send it. If it collides then maybe resend it. (this depends on you goals). If you don't need to ensure that every sample is received then just try once and go to sleep right away.
You will find that the power consumption problem is so hard that the only solution will be a network that isn't transmitting much at all. If you just try, it will probably get through. If it doesn't, you can always try again later.
If you do need to make sure every datagram gets through then you will have to do some kind of ACK scheme. In the 6LoWPAN world, you can use TCP which will keep retrying until your battery is dead. There is also CoAP which uses UDP and has a retry mechanism (but doesn't promise delivery). But every choice here will impact run-time. If you are operating for years, the impact will be in months.
Your option #2 is built into 802.15.4 hardware as CCA. The idea is that receiver turns on for 8 symbols and returns true or false. Then you can make a decision about what to do next. You can play with these schemes all day/week. But every time you do something like this you shave more weeks off the run-time. That's why I suggest to start simple for now. It will work quite well if you are trying for long run-times.
Yes, you can have multiple units receive the same broadcast (or multicast) message, if they are each configured to use the same receive address that the transmitters is sending to. (And in range of course).
Remember that each can have up to 6 configured receive addresses, so one of the ways to use the device is to configure one receive address which is unique to a given device, and one receive address which is shared with other devices (eg: all receivers, so some subset of them). The transmitter can send to a single receiver by using its unique address, or to all of the receivers in the group by using the shared address. ("Multicast" may be a somewhat better description in that it goes to multiple receivers in this scenario, but it need not go to all receivers - you can control the subset of receivers using a given shared address).
Of course, if you are sending a multicast message, it must not use ACK - because multiple receivers would try to send their ACK packets at about the same time, which would collide. (Also: since the hardware defined packet structure does not have a "source" address, the way that Enhanced ShockBurst ACK works is that the receiver sends the ACK packet to its own address, and the transmitter must be configured to receive at the address it's sending to. Thus in multicast, a given receiver's address - to which it will send an ACK - is also the shared receive address for the other receivers). So ACK must be off for multicast (as your code says).
There is not quite enough information to answer why your particular setup causes only one receiver to accept the packet. Suppose you have one transmitter T, and two receivers R1 and R2 (both using the same address). R1 is the one which "always wins". Is this the situation:
(R2 not operating): R1 reliably receives from T
(R1 not operating): R2 reliably receives from T
(both R1, R2 operating): only R1 receives from T, R2 does not
That is, adding a R1 to the second situation causes R2 to stop receiving from T. Can you verify this by starting the T to R2 test in a loop and then after a while also powering up R1, at which point R2 stops receiving; then power down R1 and see if R2 starts receiving again?
If so I'd suspect that some kind of ACK problem is involved, or there is some problem with R1 and R2 being too close with some kind of RF or antenna intererence, or they are on the same power supply and there is some interference between them through the power line.
The nRF24 family is a bit sensitive about power; sometimes you need to put a filter cap across the power input to the module; eg: a 10 uF electrolitic (observing + and -). I've had what I thought were software or config problems clear right up when I improved the nRF24L01+'s power supply.
Best Answer
It appears that the removal of the CRC is used to maintain backwards compatibility with certain other devices. If you are not using those, you can probably still use the instructions in Appendix B to make both sides "think" that they are communicating with a legacy device.