2. Training and mentoring is a reflection of your company's Engineering Management. My employer has had an easy time offshoring development to Slovakia and Bangalore because we had Tech Leads and Engineering Management make the initiative to mentor, train, and onboard offshore team employees as we would domestic employees. I've seen similar examples occur across the industry (eg. Google Pay's dev team in Hyderabad, DE Shaw's dev team in Gurgaon, Motive/KeepTruckin dev teams in Lahore & Islamabad). If Management views Engineering as a cost center (which, it pains for me to say as an ex-cybersecurity engineer, at times it can be), that's where you tend to see the relationship between offshore and onshore teams collapse, because the relationship is based on the implicit arbitrage of labor.
3. Tijuana is not a software hub. It is a medical device manufacturing hub though (all the design and R&D is done across the border in San Diego and manufacturing is done in Baja California). Also the MechE salary in MX is apparently higher than it is in Poland (~$13k/yr - https://www.salary.com/research/pl-salary/benchmark/mechanic...), Romania (~$4k/yr - https://www.payscale.com/research/RO/Job=Mechanical_Engineer...), or China (~$15k - https://www.salary.com/research/cn-salary/benchmark/mechanic...).
In fact, I think the Poland salary is overestimated as I know for a fact starting salaries for new grad SWEs in Poland (not working at MNCs) is around $7-8k/yr. Most of the software roles in MX are in CDMX, Monterrey, or Guadalajara. So in fact devs in MX might be earning more than in Eastern Europe.
I know for a fact McKinsey pays Associates 33% more in Mexico ($127k) than they do in Poland ($96k) for the exact same role, and generally McKinsey salaries take regional job markets into account when setting salaries, so I guess it shouldn't surprise me that Dev salaries might be higher in MX than PL or RO (and they 100% are higher than in UA or RU)