I have a lot that came up during my last game, but don't have the cards in front of me... So I'll ask later.
But there is a more general question:
My investigator has 1 Will. I have to do a Will test minus 1. I also have an Ally that gives my Will plus 1.
Now, how many dice do I roll?
WAIT. Thinking about it now, maybe it is always one because it's not that your skills can't be lowered to 0, it's that you always roll at least one die.
The minimum of 1 die is checked only once all modifiers to dice pool have been accounted for. So, you will end up with 1 die in this case regardless of the order of the modifiers.
See the Reference Guide, Page 11, "Test" section and follow it carefully. According to the steps listed there you:
1) Determine your dice pool.
The first bullet says start with a number equal to your skill. (In your case, 1.)
The second bullet says add or subtract the test modifier next to the icon on the card calling for a test. (In your case, -1. 1 -1 = 0.)
The third bullet says if you have an improvement token then add it. (In your case, you do not have one so you are still at 0.)
The fourth bullet says you may use ONE effect that provides a bonus, and if multiple sources use the highest bonus only. (In your case, +1. 0 + 1 = 1.)
The final bullet says if you get "additional dice" from effects they are cumulative and get added now. (In your case, you have none.)
At the end of step 1 we have dice pool of 1 for the test - you get to roll 1 die. Now go to the next step:
2) Roll Dice
The first bullet point says if your dice pool is below 1, then roll 1 die instead.
The second bullet point says if your dice pool is greater than the number of dice you have then roll as many as you have, count the successes, and reroll as many as needed counting up successes.
If you are new to the game, I strongly encourage you carefully read the Test section every time you play for the first several tests. Don't skip anything. Do the same with the Combat Encounter section and other major actions. Carefully read.
In the case of your question, the Test section makes it clear that first you do the math to compute your dice pool and then you see if you get the one die minimum in case your pool is 0 or less. The timing is clearly defined.
In other words, you intuitively got it and you probably remember reading it before. EH has some picky language, so use the reference guide any time you have any doubt about something.