The document has to define the plan and limitation year as a 12 month period. A short plan year would be an exception so the document would have additional language to indicate a specific short plan year. If the plan is effective 10/1/2018, I would expect the document to also state a short plan year of 10/1/2018-12/31/2018.
Sort of. You have to preserve the right the accounts at the time of the amendment. So you could potentially have an accounting headache for the next couple of years as the value of partially vested account in-service distributions at the time the amendment is executed must be preserved. Effectively after the term of the vesting schedule you'd have the amendment you want.