It would not so much be anger as it is fear-based aggression. He’s likely a not trusting anyone too much at the moment, after being handled by strangers and staying at the vet. Not to mention having his bits taken off. The softer poops are likely related to the stress of it all but also could be from the medications from the procedure.

Just give him some time.
It’s not unusal for rabbits to remain quite hormonal for a few weeks after neuter also.

My rabbits never had a cone after spay or neuter. Nor any fosters or rabbits I was bun-sitting after their ops. I don’t know why some vets use them. Rabbits need to be able to reach their cecatrophs and the cone hinders this. Plus, obscuring the vision field of a prey animal doesn’t make sense to me…

I would ring and ask if you can forgo the cone as it’s causing him distress. If he had glue, not stitches, he would pull anything. You would have to keep a close eye he doesn’t lick there too much though.

This information is from the House Rabbit Society on after-care:

What pre- and post-operative care should one give?
Some rabbit people give their rabbit acidophilus for a couple of days prior to surgery, just to be certain that the digestive system is functioning in fine form. But don’t change the diet it any way during this time.

After the surgery, ask your veterinarian for pain medication, especially for a spay. If you choose, continue giving acidophilus until the appetite has returned to normal.

Inspect the incision morning and evening. After a neuter, the scrotum may swell with fluids. Warm compresses will help, but it is nothing to be overly concerned about. With any sign of infection, take the rabbit to the veterinarian immediately.

After surgery, keep the environment quiet so the rabbit doesn’t startle or panic, don’t do anything to encourage acrobatics, but let the rabbit move around at her own pace– she knows what hurts and what doesn’t

Some veterinarians keep rabbits overnight. If your veterinarian lets you bring your bunny home the first night, note the following:

Most males come home after being neutered looking for “supper”– be sure they have pellets, water, and some good hay (good, fresh alfalfa is a good way to tempt them to nibble a bit)
Most females want to be left alone, are not interested in eating at all, and will sit quietly in a back corner of the cage (or wherever in the house they feel they will be bothered the least). Try not to pick up or bother her much for the first 4 days. Females take it harder.
The following morning, or at latest by the next evening, it is important for the rabbit to be nibbling something. It doesn’t matter what or how much, as long as she is taking in something, so the digestive tract won’t shut down. If she isn’t, tempt her with everything possible, and as a last resort, make a mush of rabbit pellets (1 part pellets, 2 parts water, run through blender thoroughly, add acidophilus, and feed in pea-sized bits with a feeding syringe through the side of the mouth with them sitting upright). But this should not be necessary if you give them their favorite Italian parlsey, a piece of banana, dandelion green, cold, fresh greens, washed, and you hold it for them. They should start nibbling on it.

Occasionally a female will pull out her stitches. Get her stitched up again, and then belly-band her by wrapping a dish towel around her whole middle and binding that with an elastic bandage wrapped snugly over it. If she can breath normally, it isn’t too tight.

For the record, my own rabbits weren’t given pain meds after neuter. I did have some for the girls post spay, but not after my first girl was spayed. Luckily, she recovered quickly. Vets will often give a long-lasting injection after op. 

A way to check the site without handling your rabbit is to try get him to stand for a treat. If he is in a cage or pen, he can use his front paws to hang on while standing up. Then you can take a peek at the area to see if all looks good.

I personally didn’t restrict my rabbits too much after neuter, but made sure there wasn’t anything they could jump up onto. I would keep your bun away from the cat longer because if he gets too active or tries to mount her, that could affect the healing site.
I think letting him out onto the floor under supervision would be alright. Give him something to keep occupied.

I got my boy neutered last month. My vet (rabbit savvy) didn’t give us any instruction either. Also, no pain medication. My boy got very bad shape the first day and night but bounced back the next day. He normally won’t let me hold him, so I couldn’t check his surgery. Sometimes, I held a carrot top to make him stand so I could see if any obvious sign of infection. I called the vet and she said I didn’t need to check the site.

My boy was also free roam the whole house. But I limited him on the cage the first day. And I limited him to jump or run crazy up for 10 days. On day11, I let him play with another bunny (not completed bond). My vet said 2 weeks is good enough, especially if you are going to introduce another bunny. After neutering, my boy doesn’t have any sign of aggressive behavior since he never had. But I notice he looks sad. His personality isn’t the same.

Hope this help answer your question.