Well, weight is entirely due to the fact that a neutered anumal just needs to be fed aboyt 10-15% fewer calories! Their metabolism sometimes slows. So just feed a bit less and you will have no issues with weight gain. Wooly coat (or cotton coat) is FAR MORE an issue of 1) overweight dogs and 2) genetics. I hjave two nuetered males with perfectly silky coats that I keep lean by watching their diet and plenty of exercise -- a good combination for any dog regardless of whether it is neutered or spayed.
You are right about the health issues. An unspayed bitch is at regular risk of pyometra, a womb infection that only occurs in unspayed females. The same for ovarian cancer and tumours. Also you need to keep her in for 3-4 weeks twice a year -- TOTALLY inside -- when she's in heat, during which time she will spot blood. And a pregnancy always carries its own risks, with mixed breed puppies very hard to home if that's the result of an unwanted/unexpected breeding. Also there's been a serious rise in incidence of stolen dogs in the UK and Ireland with people who take them selling them on for breeding, often into hellish 'puppy farms'. Cavaliers are frequent targets -- this is considered one of the fastest rising crimes in the UK and has been featured in several stories recently in the UK.
Talk to Dog's Trust -- they can give you the facts on spay/neuter.
I also strongly recommend this article:
http://www.marvistavet.com/html/body_the_canine_spay.html
I spay/neuter all my dogs and have tags that indicate they are neutered on their collars to discourage anyone trying to steal and keep them.
On this issue, here's what Dog's Trust advises, from their website:
I am not sure why UK breeders are so prone to discouraging pet owners from neutering when thousands of unwanted dogs, including cavaliers and mixes, are put down annually in the UK -- and there are so many health benefits to neutering. In most countries pets are always sold on restricted ownership clauses which REQUIRE neutering so that the dog is not bred. This isn;t a criticism of the many great UK breeders so much as it is meant to indicate many excellent breeders around the world are very strong advocates of the many benefits of neutering and even require it.
Personally -- from a long while now working in dog and cat rescue -- I think the average pet owner finds managing intact males or female cats or dogs simply too much extra work with some unwanted side affects (the blood spotting, marking, humping behaviour, bitches in heat attracting in every wandering male dog for streets around for weeks on end).
Many vets strongly advise spaying females due to the risk of death and high cost of emergency treatment from pyometra alone. Also, even if the coat does coarsen slightly, this seems a very minor cosmetic negative set against many, many positives.