Questions regarding an upconverter topology (beating behavior, P1dB and compression)
It's been 25 years since I messed with this stuff. RFMD had some nice modules, and some excellent descriptions (but have since been swallowed up.
Your up converter is a type of I-Q modulator, It comprises a double balanced mixer on the left for the Inphase signal, and an identical mixer on the right. Your local oscillator is fed into LOi and a 90degree phase shifted copy of that is fed into LOq.
Each mixer generates a &quot;double sideband&quot; output if the input signal has no DC component, ( if there is some DC component, then you will generate some amount of carrier signal at LO frequency). If you look at a double sideband signal on a scope (assuming a sinusoidal input) then the envelope of the modulated RF signal has a a pair of criss-crossing overlapping sinusoids, perhaps this what you mean by &quot;beating&quot;  ( &quot;beating&quot; is a bit vague, all mixers create &quot;beat&quot; frequencies).
If you were to drive a sine wave into just one of the inputs, and observe the output on a scope, it would look identical to if you just input that same sine wave to the other input , however both outputs are different, one is phase shifted by 90degree, (i.e. if the LO were 100MHz , then one signal (the Q usually) would be shifted 2.5nS later.
The magic happens when you add the modulated in-phase output to the modulated quadrature output, as parts of the signal add and other parts subtract.  So if you add a modulated Q output to a modulated I output then you get just one sideband. if you subtract a modulated Q output from a modulated I output you get the other sideband. The easiest way to get the addition is to apply the  signal to XBB,i and a 90 deg phase shifted copy to XBB,q  , The easy way to get the difference is cross one of the wire pairs to either  XBB,i or XBB,q .
You can do a whole lot of math to prove the adding / subtracting effect , or you can make yourself a simple spreadsheet and play with that. There is a bit more info here : https://www.rfcafe.com/references/electrical/quad-mod.htm
and a whitepaper here https://www.everythingrf.com/whitepapers/details/3186-iq-image-reject-single-sideband-mixer-primer which goes here https://cdn.everythingrf.com/live/IQImgRjctSideBndMxr%20(marki).pdf
This from minicircuits is not that helpful. https://www.minicircuits.com/pages/pdfs/an00009.pdf
Some Excel plots of the mixer behaviour : input connected to inphase only, note the dashed violet lines for reference. The green output of the mixer is symmetric, each &quot;burst&quot; is the same

Input connected to quadrature only, note how zero crossings are now peaks (dashed violet lines) and the green output trace is now antisymettric (each second burst is inverted): 
If we put the same signal into both inputs, then the output is the sum of the above, i.e. 1.41* larger with a phase of 45degree. (On the green output the alternate bursts are mirrored in time and amplitude) 
To get a single side band we need to shift one of the signals by 90degrees, this is not the same as &quot;shorting inputs together&quot; as one input lags the other. note how the green combined signal is slower than the LO, LO=10Hz, signal=1Hz USB=9Hz
To get the other sideband, just invert one signal (i.e. cross wires to one input), note how the green combined signal is faster than the LO LO=8Hz, signal=1Hz USB=9Hz