I’m sorry to hear about all the problems you’ve been having lately. It sounds like you’ve really been going through a difficult situation, much of which was completely beyond your control. Try not to get too discouraged by all the adversity. You can’t help it if a serious illness laid you up at the worst possible time.
Cycling problems are very commonplace, Deb. In fact, it is so common to lose specimens to ammonia and/or nitrite spikes when cycling a new aquarium that the phenomenon has a name — such deaths are ascribed to "new tank syndrome." Once your aquarium has cycled properly and the biofiltration is well established, you are going to find it much easier to keep seahorses and other marine fish healthy and happy in a mature, stable aquarium.
When making water changes to combat ammonia spikes, it’s more important to get clean, freshly mixed saltwater with zero ammonia and zero nitrites into the aquarium quickly than it is to make sure that the water is well reached beforehand. Dangerous ammonia levels are an emergency situation, so don’t hesitate to take shortcuts if it proves necessary. Just go ahead and complete your water change if you are up to it before you leave for your trip and hope for the best. Your biofiltration may well have adjusted to the heavier bioload by the time you return, and then you’ll have a cycled aquarium all ready to go waiting for you when you get back. Once that happens, all your problems will be minimized.
Hang in there in the meantime, Deb. I can assure you better days are ahead for you and your seahorses.