@Leviuxe , I think you are making the situation more complicated than it needs to be. The treaty, the protocol of 2003 and the technical explanations 2003 etc. cover a whole bunch of conditions, complicated situations etc. -- only because they are trying to cover all possible scenarios in generalities , being specific would limit their options downstream.
In your particular case , I see it this way and without giving any weight to TN or L or H etc. type of work visa:
1. you a Mexican citizen, have entered US with a work visa
2. You live in California and will be here as long as your visa is valid ( 3 yrs).
3. You have home etc. in Mexico
4. Your employer in the USA has a permanent establishment in the USA ( i.e. it is an US entity).
For US purposes,
1. you are Non-Resident Alien till you pass the SPT -- 183 days presence in the USA ( counting current year, previous year and 2nd previous year presences for any purposes )--- and taxed ONLY on US sourced income ( i.e. for work performed in the USA and/or US connected income). You report this income, itemized deduction etc. on form 1040-NR.( Not supported by TurboTax)
2. Once you pass the SPT, you are a Resident Alien for Tax purposes ( immigration status is still Alien with work visa ), you are taxed on your world income and you file your return on form 1040.
For Mexico purposes what I understand from PriceWaterHouseCoopersLybrand ( PWC :(
"Resident individuals are subject to Mexican income tax on their worldwide income, regardless of their nationality. Non-residents, including Mexican citizens who can prove residence for tax purposes in a foreign country, are taxed only on their Mexican-source income. "
Thus there appears to be a way for you to avoid double taxation. Else, you can always use Foreign Tax Credit mechanism ( but it would mean that in your specific case you would have to use Treaty conditions to resource your US income to Mexico --- complicated but often done).
Does this help ?
Is there more I can do for you ?
Adios Senor
pk
