In terms of the potential scope of lost rep opportunity for contributors, do people besides me not upvote answers that they can't easily verify?

Yes, I agree. I upvote only answers which I can verify (AND which I consider to be useful, gives additional information to previous answers, ...)

Do others agree that on non-obvious or non-trivial answers, it's important to include the results as proof?

In general, yes. (of course there're also exceptions)

If so, is there something we can do to educate and encourage authors to do that?

I've experience mainly with Excel questions. There are two types of questions here:

• questions asking for formula (worksheet-function):
  here, adding a screenshot which shows the formula and the correct result could be a quick win, this would already increase the quality of the answer to an acceptable minimum, however that still doesn't make it a good one (see this question too: Should I write detailed answers to trivial questions?)
  When I see answers with the formula only, or even worst: "try this...", I'm tempted to flag it as comment only
• answers containing VBA: for some people on this site writing (short) macros is a quick win for answering, even if OP asked for a formula, or the questions is of a low quality.
  I find very few good quality questions asking for code (containing: what do you need? what you've tried?) and really hate when people comes here just asking for the code (Is there a macro, which can...?)
  As I generally don't agree with the approach, I'm more strict here: upvote only answers which are answers of a good question and also add some explanation to the code.
  For the original question regarding education: for me adding a proof here is a low priority, before they should understand: answer good questions only; consider coding level of the OP (if you write a code for a novice in VBA, then also include link for VBA tutorials to teach him how to use it).

In terms of the potential scope of lost rep opportunity for contributors, do people besides me not upvote answers that they can't easily verify?

Yes, I agree. I upvote only answers which I can verify (AND which I consider to be useful, gives additional information to previous answers, ...)

Do others agree that on non-obvious or non-trivial answers, it's important to include the results as proof?

In general, yes. (of course there're also exceptions)

If so, is there something we can do to educate and encourage authors to do that?

I've experience mainly with Excel questions. There are two types of questions here:

• questions asking for formula (worksheet-function):
  here, adding a screenshot which shows the formula and the correct result could be a quick win, this would already increase the quality of the answer to an acceptable minimum, however that still doesn't make it a good one (see this question too: Should I write detailed answers to trivial questions?)
  When I see answers with the formula only, or even worst: "try this...", I'm tempted to flag it as comment only
• answers containing VBA: for some people on this site writing (short) macros is a quick win for answering, even if OP asked for a formula, or the questions is of a low quality.
  I find very few good quality questions asking for code (containing: what do you need? what you've tried?) and really hate when people comes here just asking for the code (Is there a macro, which can...?)
  As I generally don't agree with the approach, I'm more strict here: upvote only answers which are answers of a good question and also add some explanation to the code.
  For the original question regarding education: for me adding a proof here is a low priority, before they should understand: answer good questions only; consider coding level of the OP (if you write a code for a novice in VBA, then also include link for VBA tutorials to teach him how to use it).