Visual understanding of EM fields within a rectangular metal container

915MHz has a wavelength of about 300mm, or one foot. The container has internal dimensions of many many times this. This means the standing wave pattern, resulting from the inevitable interference, is multimode. Many different modes can be excited.

While you can guarrantee that there will be zero tranverse electric fields adjacent to the conducting walls, you can guarrantee very little else. In theory, accurately known dimensions would allow you to compute what modes would be excited. In practice, the slightest irregularity in the walls will cause all your predictions to be wrong. The other 'off' antennae will complicate the geometry no end. Points of high field and low field will move around as the container walls flex, as the contents moves, as the frequency shifts slightly.

With a reasonable degree of certainty, you can say that a low field spot with one antenna running is unlikely to still be low with another one in use. If your red dots/balls are transceivers to be communicated with, then you're likely to be able to talk to all of them, just not necessarily all at the same time, pick the right antennae for each transceiver.

As you don't know for certain which combination of the many modes possible you are driving, the antenna impedance will be unpredictable.

For industrial heating, it is common to choose a frequency and box dimension that supports only a single mode. This way, the field strength within the box can be accurately predicted.

I cannot give you a complete answer and you would need some kind of 3D electromagnetic simulator. I have used HFSS (a FEM simulator) and CST (a FDTD simulator), but both of these are extremely expensive. Based o a quick googling, there are some free alternatives, but I have no experience on any.

The device you are looking at is called a Cavity resonator. The cavity resonator is a conductive box (like yours) where the waves transmitted by your antennas "bounce" of the walls. At certain frequencies, the length of the box wall is an exact multiple of half of the wavelength. At those frequencies, the waves can bounce back and forth in the resonator very well. That is called a resonance. The list below is a list of frequencies where the resonance happens. The subscript numbers correspond to the number of half wavelengths that fit inside your box at that frequency. The lowest resonance mode is called \$TE_{101}\$ and that mode can fit (exactly) one wavelength in two directions and none in the third direction. In your resonator, the TE101 resonance happens at 64 MHz. See Compact cavity resonators using high impedance surfaces for field plots of a TE101 resonance

I used the formula for resonance frequencies of a rectangular cavity and listed below all modes that are within 1 MHz of your design frequency. As you can see, there are multiple different resonance modes near your frequency and it is not easy to tell which mode you are exciting. You might even end up exciting several modes at the same time.

$$f_{16-14-2}: 915.0 MHz$$ $$f_{6-14-3 }: 914.5 MHz$$ $$f_{7-14-3 }: 915.6 MHz$$ $$f_{6-13-5 }: 914.5 MHz$$ $$f_{7-13-5 }: 915.6 MHz$$ $$f_{14-12-6}: 914.2 MHz$$ $$f_{15-11-7}: 914.4 MHz$$ $$f_{11-10-8}: 914.8 MHz$$ $$f_{14-6-10}: 914.2 MHz$$ $$f_{1-3-11 }: 915.7 MHz$$ $$f_{2-3-11 }: 916.0 MHz$$ $$f_{11-2-11}: 915.2 MHz$$ $$f_{14-1-11}: 915.1 MHz$$ $$f_{15-0-11}: 915.4 MHz$$