If your ammonia has dropped, good. Especially as it's only 12-ish hours since you added the last dose. Stick to the usual dose of ammonia until your nitrite has dropped. If you increase the amount you add now, you'll just get a much higher level of nitrite. This is why the current thinking is to add a low dose until the nitrite eaters start to grow, then increase it when there are enough nitrite eaters to cope with the lower dose.
If you have some Nutrafin Cycle, you may as well use it, it won't do any harm. It is unlikley to help with the nitrite eaters but it may help with the ammonia eaters. It is difficult to make these bottled bacteria with nitrite eaters that are still viable when it reaches your tank. Ammonia eaters are more likely to survive in the bottle.
As for a water change, I'd do as big as you can manage. But don't add any ammonia or Cycle till your usual time. Nutrafin Cycle - some of these products contain ammonia but I don't know if this is one of them. (They work by getting you to do a fishless cycle without realising that's what you are doing). It might be worth adding a dose, then testing for ammonia to check before adding any from your ammonia bottle.
If you have a combi boiler, using hot tap water is the easiest way to get the water warm. It's when you have a header tank in the attic feeding water to a hot water cyclinder that there is the potential for problems. It's not copper getting in the water as most people think, it's whatever is lurking in the header tank. Dead rats etc are not unknown. That's why I heat water in a kettle for my water changes - we've had several wasps' nests in the attic and the header tank usually has dead wasps in afterwards, possibly wasps covered with insecticide. That was the main downside to all the water changes I did during my cycle. It took 1.5 litres boiling water (one kettleful) plus 3.5 litres icy cold tapwater in a bucket to get a temp of 30o. As I was doing 95% water changes, that was 5 kettles' worth of boiling water to make 25 litres.