The ADU would then be used as a long-term rental.Does this qualify as a “like-for-like” property exchange even though I’m technically not buying another property and does a 1031 apply?

Find a lawyer and issue a lease violation for other issues such as excessive noise and furniture removal.

Alternatively, I have spent the last decade doing technical work in cybersecurity, and can land a job between $140k-$190k (depending on where in the US). 

If the damage is minor, again I think the tenant can simply claim the quality of the appliance was poor and they have the door-seal quality issue to further back up this claim.

You technically CAN break it up on Schedule E; however, it is not the best thing to do for audit purposes or an actual breakdown.You should have an operating/partnership agreement together and you should have a dedicated bank account for the property.  

. - Just be prepared, because a motivated seller typically has a property with issues you need to be sure you can solve.

Although many properties with a specific zoning code are technically eligible, they would be subject to an extensive waitlist that could last years.

4.Tips to avoid similar issues in the future?

its an insurance issue. not much else to do.. 

Compliance states there is no way to remediate this unless a new file is created. lender credit is 6900 which is the seller concession and no COC can be issued for “incorrect” lender credits.