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The accountable plan is up to the payer to decide IF they are keeping track.  They may not be. This is not a requirement.  They may just lump it altogether which is perfectly acceptable as well. So if they are not tracking it separately, they would be in the right to include all of it on your 1099.  This means you would need to deduct your expenses.  

In the end, your total profit is the same.  It does not change how much you profit since you will be deducting what you spent on the supplies that they paid you for and included in your 1099. 

Yes, if you are self-employed, then all of this would be included as expenses for your business.

Your expenses would include mileage (ONLY that which is directly related to your business), or if you prefer to use the actual expense method gas and oil changes and vehicle expenses, supplies, phone, advertising, etc. Basically any expense that is directly related to your business NOT YOUR PERSONAL expenses would be deductible. 

You do not need to separate your expenses based on client, instead you would group them together like 30 bottles of dawn dishwashing liquid used for 20 clients would all be one expense.  

If you have a W-2 job, then the expenses incurred to go to work as an employee are NOT deductible neither are expenses incurred to do your job as a W-2 employee.