Electronics problem (airbag and anti skid warning, ventilation not working)

[tman]

I am having some electronics issue and doubts about the fix that my mechanic proposes. The car: 2011 XC90 Summum US version. The symptoms (in chronological order):

• Ventilation and AC stop working, no more lights on panel
• Passenger Seat Airbag malfunction warning appears
• Ventilation and AC work again (after 2 or 3 days)
• Mechanic resets or recalibrates something but warns that this might only temporarily fix the issues
• Passenger Seat Airbag malfunction warning disappears and does not return anymore (during the 6 weeks since)
• Anti skid system malfunction warning appears
• Anti skid system malfunction warning disappears and does not return anymore (during the 5 or so weeks since)
• Ventilation and AC stop working again and does not work anymore for 5 weeks
• Ventilation and AC starts working again

The mechanic proposes the following solution:

• Replace the control console (part 31346604) with a listed price of $923.24 to fix the ventilation and AC issue
• Replace the complete OWS thingy (part 30767446) with a listed price of $1,479.60 to fix the airbag warning issue

My problems with this approach (I don't understand much about cars, but a bit about electronics):

• How does this fix the issue with the Anti skid system malfunction warning? (I will ask the mechanic this tomorrow)
• Is it really necessary to replace the complete OWS thingy?
• Is it really necessary to replace the control console? The ventilation working, then not working, then working again might just as well be a bad solder joint or loose wire somewhere.

How does a mechanic diagnose such issues and come up with a solution? Do you think the proposed solutions are too drastic and the issues could be fixed with less expensive replacements? Do you think the mechanic has no clue what the problem actually is and just hopes replacing the biggest part that relates to the issue might solve the problem?