If you think the distance is due to one of these incidents, my hunch, is that it was the pool request. Kids bite. My daughter bit a friend when they were toddlers, and it mortified me. If it were an issue where your daughter had a pattern of biting, that might be different. But, I kind of doubt they are upset about a bite.
I have to agree that the request of arms length reach of your daughter might have been taken to mean they aren't responsible. I'm sure that wasn't your intent. My kids swim at friends' houses and I will ask about the pool rules or provide a life jacket or something if I can't be there. Whatever the case, your families had been friends for awhile and you might have just told them you don't trust their judgement in their own pool. Having kids around water is scary. We have a pond, and I would never let kids swim without me right there(within 10 ft or so), or have them wear life jackets. I realize there are parents out there who maybe aren't so careful. But you know these parents pretty well. That request may have been a little much in their opinion. I absolutely think that as a parent, you have to protect your kids and asking about guns in the house or if an adult will be present by the pool is acceptable. But, if you making "demands' on the distance, you need to just be there yourself or not let her swim. I agree with the poster who has a pool. Ours is a pond, but if I was told I had to be within arm's reach, I would have to get in the water. That ain't happening every time they want to swim. I will sit right next to the water, but not be in it.
Now, it could be like others said. Their daughter has made friends with others and may not have the desire to invite your daughter over, much anymore. It happens. Find some other girls to invite over and maybe their friendship will rekindle again in the future.
If you want to get to the bottom of it, flat out ask again. This time, bring up the pool comment. Tell them you didn't mean to offend - see if that sparks a conversation on whether or not that was the reason for the distance.
Good luck :)