Was this plan covered by a fidelity bond?

On both a 5500 and an SF, the opening wording to question 4 or 10 (and their a through i or n subsections) says, "During the plan year:"

So I don't think you're properly answering the question if you obtain it after the year-end but still suggest it was covered (unless retroactive).

And as for the "part of the year" scenario - I think it's reasonable to say that "the plan year" does not specify the entire plan year.  Meaning, if you got your coverage on December 31, then the answer of "yes" is a true answer because during the year it was indeed covered.  Heck, I'd suggest you get to leave "yes" as your answer if your policy expired on December 30.

(Recall on the SF, you're asked if the plan had loans, even if the they're all paid down to zero by 12/31, so you still admit there were loans, even if it wasn't all year long.)

As for the dollar amount, I'd use the largest amount of policy in effect during the year.  Or if the plan just uses an inflation guard, I'd use the 10% BOY amount.

3 hours ago, Peter Gulia said:

How does one answer this question if a fiduciary obtained the insurance before the administrator signed the Form 5500 but after the reported-on plan year ended? (Assume the fiduciary does not buy retroactive coverage.)

You answer "no" (I have seen many people answer yes even without retro coverage, but it would be technically incorrect)

3 hours ago, Peter Gulia said:

Does the past-tense “was” refer to any time in the past?

Fixed at the beginning of the plan year (ERISA 412(a))

3 hours ago, Peter Gulia said:

How does one answer the question if the fiduciary obtained the insurance during the reported-on plan year (so the plan was uninsured for some portion of the year)?

Technically, it should be "no" if you at any point during the year had a plan official handle or deemed to handle funds or property of the plan without adequate coverage*.  That said, I have had both IRS and DOL tell plan sponsors to get a current bond if none existed, without requiring retroactive coverage.

3 hours ago, Peter Gulia said:

For the line that calls for a coverage amount, does one report the amount that applied as at the beginning of, or the end of, the reported-on year? Or is it the highest coverage amount on any day in the plan year?

You report the amount of coverage.  This could mean the face amount of the bond, or another amount if the bond has a rider that applies 10% at the time of the claim. So, you could have a face amount of $20k, but with a rider to cover 10%, in which case you would enter the greater of $20k or 10% of BOY assets.

3 hours ago, Peter Gulia said:

Am I right in guessing the usual software restrains a user from answering both Yes and No?

Yes.  Its one or the other, and you cant file electronically if you answer yes but do not enter a bond amount.

* edited for context.

If you want TMI, get your favorite beverage and read FAB 2008-04 (18 pages).

If you want to see what an EBSA investigator will look at when reviewing a plan's ERISA bond, peruse the Bonding checklist (3 pages).  I believe anyone going through the checklist will think of at least one client that likely has a bond that is not compliant.

2008-04.pdf Bonding checklist - EBSA Fiduciary Investigations Program.pdf