Pointers aren't numbers at all - they're number-like, and they can be added, subtracted and converted to formal numbers, but they're definitely not signed integers.
However - in C, a pointer type can (and should) indicate what it points to. A char* isn't a signed pointer, it's a pointer to memory which is to be interpreted as char. If you change the pointer type, you're still pointing to the same memory, but it will be interpreted differently.
Your strcpy function will work if you give it the types it expects. I don't see anything wrong with your code - it will run, despite the warning. Change unsigned char to char and it should work with no warning at all.
If you need to convert them to unsigned 8-bit integers to transmit them to the LCD, this would formally be considered part of encoding them for transmission, and should be done as part of transmission itself, not universally throughout your code. They're just char up until you need them to have a specific format to send them to the LCD. You can "convert" with a cast:
// Here's a normal char
char c = 'Q';
// Here it is as unsigned: implicit cast
unsigned char d = c;
// Here it is again as unsigned: explicit cast
do_something ((unsigned char) c);
Store them as char because that's what they are - characters. Convert them when it's time to transmit. I did say not to do this, earlier, but specifically for mathematics - it can change the meaning of the number. What it will do is take the binary encoding of the char and just re-interpret it as an unsigned char. That's exactly what you want to do, so it's okay.
As for the math - I don't know, that might be platform specific. I'm sure you can do it. You might have to tell the compiler to link to the math library - with GCC and similar compilers, this is done by adding -lm as a compile option.
I should include the usual admonition against floating-point math in microcontrollers - but I've definitely encountered a few times when it was quite useful, so if the MCU is powerful enough I really can't complain. Just make sure it really is the best way to accomplish what you want.