Heather, heat pads will not burn frogs when used in conjunction with a thermostat like they should be. Only heat pads that are unstatted or set at the wrong temp will, this will apply to them being used on the wall as well. Also proper monitoring of temps is essential, which should be done by a digital thermometer directly ontop of the mat and not an analogue thermometer. Also, if conditions are kept dirty and the frog stays on the mat the frog may develop an infection that might look like a burn (though I've never seen this) frequently cleaning out the burrow and changing substrate as needed will prevent this. I just wanted to clear that up because keepers that use them correctly shouldn't worry about their frogs being burned. That being said, above heat sources are better because it's more natural and they warm the ambient air temperature, so are best for cold houses.
You don't need to use calcium plus with a different supplement for D3 btw, it has it in it

The normal one has medium levels, which I the one I use and there's also a high and low one too.