You are soooo close to being correct. However, close to correct is wrong. The tax code for unreimbursed employee business expenses has not changed significantly, but you have made some serious errors.
The Ferderal per diem rates are "maximum" rates. That doesn't mean that an employer can't pay more. It means that they are the maximum amounts that an employer can pay under an accountable plan without requiring the employees to prove the expenses.
Keeping detailed records does NOT make your per diem taxable if it would not otherwise be taxable.
You can NOT deduct $200 of expenses if you are given a $100 reimbursement. What you can deduct is the difference between your expenses and what you are reimbursed for. It has to be done on an annual basis -- you add up all the expenses for the year, subtract all the reimbursements, and deduct what is left.
If your employer gives you more per diem than the published maximum rate - for even one day - you may have to account for all of your expenses for the entire year.
There is no requirement for an employer to pay the same per diem to Jr's as to Sr. Techs. In fact, they are not required to pay per diem at all. They can choose to offer different reimbursement schedules to different job classifications if they desire. This does NOT make the per diem taxable as pay as long as the employer doesn't exceed the maximum rates and doesn't pay for days when te employee is not eligible.
Withholding per diem for a sick day - while a despicable practice - does NOT make the rest of the per diem taxable as income. Employers MAY (but are in no way required to ) reimburse travel and living expenses for days that you are away from your tax home. If they have reason to believe that you were not away from your tax home, they are not under any obligation to cover expenses. Since per diem is only treated as tax-free reimbursement if it is paid under an accountable plan, you actually do have to prove that you were away from home on business in order to be eligible. Being "sick" on a Friday and/or a Monday brings that into doubt. Even if you can produce a hotel bill for those days, the "for business" part can still trip you up. The best bet is to not work for a company if they use this tactic when it is not reasonable. Unless you are abusing the sick days, it is just as reasonable for the employer to assume that you were actually sick and to treat the day like any other day off and pay the per diem. They're
pretty much within their rights either way, but that doesn't mean that you have to accept employment with them.
There is no statutory limit as to the distance one must be away from home to receive per diem. All that is required is that you be "away from home". Generally, employers use 50 miles, but the distance could be more or less depending on the circumstances. If you are working 8 hours per day and are only 60 miles from home, you have plenty of opportunity to return home each night, and your employer may decide that you do not need per diem. Or, you could be working 13 hours a day and be only 40 miles from home, but returning home each night is impractical or unsafe. In this case, you could be reimbursed for your living expenses without breaking any rules.
There are no time of day limits on travel days. You are entitled to the lodging for any day when you spend the night away from home. The first and last day, you are entitled to only 1/2 of the meals and incidental expenses. To apply this practically, my company pays a full day of per diem starting wwith the first day of travel to a job - and no per diem on the last day of travel home. This actually overpays the employee by 1/2 of the M&IE portion on the first day and underrpays him by the same amount on the last day of travel home - so it is a wash and easier to account for.
http://www.irs.gov/pub/irs-pdf/p15.pdfPage 11 describes briefly the rules for an accountable and non-accountable plan.