Welcome to the BB community Sam! That’s a lot of information, so definitely good to come to the forums and get some recommendations. I am not personally experienced with bonding, but I’ll share what I know based on research and what I’ve read through other’s experiences. Other members will probably chime in as well.

Yes, male parts can drop later than 6mo. Unusual, but possible. Wick’s dropped at 8mo.

You should separate them now, knowing the uncertainty is there. Also, they should stay separated at the least until after your other rabbit’s spay, plus 1-2 months in order to let her hormones fully regulate//dissipate. Following a spay, hormones go through a spike that amplify “bad” behaviors, that make interactions with a potential bonding partner very unfriendly potentially. Any bad air could lead to difficulties down the road when you want to properly bond them. The ideal, though it is not time and space-wise, is to have both your new rabbit and your female fixed, wait 2 months after your new rabbit is fixed, then begin doing dates, short interactions, etc.. Until then, they should be separated to prevent potential hardships. While they seem to get along now, it could be Spartan’s hormones have not kicked in yet, so inevitably a young rabbit will have kind interactions with any other rabbit. If/Once the hormones do kick on, the cuddles may turn into fights, aggressive humping, etc., so separation now is best.

It’s concerning that Spartan is very skinny but this may explain why he/she eats so much, as he/she has low food security (i.e. unsure when next meal will be, so stuff in as much as possible). Hopefully as Spartan realizes you’ll be giving he/she appropriate quantities of food regularly, he/she will be less of a glutton. Gaining weight is a priority though, especially because you want he/she at a healthy weight before any fixing procedures.

Since your vet ruled out blindness, I’d attribute the “blindness” you’re seeing as rabbit tunnel vision. This happens to Wick frequently that I too have had talks with my vet about potential neurological or vision problems. In the end, Wick simply snaps into a “daydream” mode very fast, and has very selective attention. If he’s fixated on something, he won’t notice a frying pan slamming on the ground, a foot about to walk into him, etc.. Being in a new environment, there are a lot of new cues and items of stimulation Spartan is trying to take in, so fixating on one and tuning out others is natural and more healthy than getting overwhelmed by turning his/her head at every new stimuli. Give some more time, take it slow, and as Spartan settles in, you may see a diminishing of the selective attention. Wick still has it though, but Wick is also quite dumb, haha.

In terms of your rabbit ownership and set-up (I noticed you mentioned a neutered male; current ownership? Is the male housed with the unspayed female?), can you give more details?