What a great exercise.
At first I thought this was easy - a simple lookup() (or XLOOKUP()) using two tables - one for the Master word, and one for the calculations:

The formula in Puzzle::A3 is:
=XLOOKUP(A1,MASTER::1:1,MASTER::2:2,0,0,1)+
XLOOKUP(A2,MASTER::1:1,MASTER::2:2,0,0,1)
Breaking this down, it's a series of two XLOOKUPS, where:
=XLOOKUP(A1,MASTER::1:1,MASTER::2:2,0,0,1)
takes the value in cell A1 (blank in this case) and looks it up in Master::1:1 (row 1 of the Master table). For any match, it returns the corresponding value in Master::2:2 (row 2 of the Master table).
If no match is found it returns 0 - if you don't need this control, you could use a simpler LOOKUP() that will throw an error when no match is found, which might be useful to make sure the calculation is valid (i.e. only uses valid letters_.
Either way, this returns the corresponding 'score' for the given letter.
This result is added to another similar calculation that works on the second word (A2).
Copied across, this gives you the calculation that works out each letter pairing.
Then comes the gotcha. In the case of A + L we get a result of 12, so that needs to be carried over, and requires an extension of the formula.
To do this, I added another row that takes these values and gets the answers you're looking for:
Cell A4:
=MOD(A3+QUOTIENT(B3,10),10)
QUOTIENT() works out the 'carry over' value from B3 ÷ 10 and adds that to A3 (0 in this case), then divides by 10 again to get the remainder. Filling this across, and editing H4 (which doesn't have a QUOTIENT() to calculate) gets:

Now Puzzle::row 3 can be hidden since it's not needed and helps clean up the sheet. You can also use conditional formatting to hide the 0 values.
What isn't clear is the addition/subtraction piece - are they always in this order? In which case you can duplicate the formulas and just replace + with -.
If you want a more dynamic approach, where you can enter + or - in a field and have it filter through, that's a little more work, but still doable.