Two rule clarifications…
With page references if you can musters some…

1. Compel - Can a model be compelled to move into deep water and forced to make a swim test? Rules don’t specifically prohibit water/swim tests.

2. Foresight - Confused on Elrond’s foresight as well. Rules mention plural “dice”, so I was assuming both sides roll dice before Elrond determine which die to alter, his or his opponents. My opponent points out that the word “dice” and “die” are used interchangeably in GB and the the rules say “has” not “have”’ meaning a single die is rolled at a time. Elrond then has to decide whether to alter the first die before knowing what the 2nd die result is. Which is correct?

As far as Elrond's foresight goes, I'll say this. It is true that the main entry for priority in the rulebook don't specify that both players role their priority dice simultaneously. This has always been the way it was done, but it isn't strictly required by the black letter rule. It is also true that Elrond's Foresight rule can be read in the singular to mean that Elrond's controlling player can alter the result of his own die (it does say the effect may only be used on the controlling player's die and not on either players') after he has rolled, but before the other player has rolled. However, this is not required by the rule that he do so as your opponent argues; no such precondition is stated. The rule only requires that the foresight points be expended "after" the controlling player has rolled his die. Nothing prohibits the controlling player from waiting for the opponent to role before deciding to expend foresight points. So long as the points are used "after" the controlling player roles and during the priority phase, it's in accordance with the rules.

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

Thanks for your insights. I think I can agree with that. Any thoughts on compelling your opponent's model into deep water forcing a swim test?

For compelling into deep water, I'm not sure. As you say, the rule does not strictly forbid making the model take a swim test like it does with other sorts of chart tests. The rule does state that you can make the model enter difficult terrain, but deep water isn't classed as such for any model like shallow water is.

If this came up in a game I was playing in, I'd probably say "the rules have created this interaction and didn't otherwise prohibit it, so it's fair game." I know that sort of logical can lead to situations where the rules will do illogical things that need FAQs to resolve (like cavalry getting their charge bonus while being in deep water), but I chalk that up do the nature of bugs in these sorts of games.

What are your thoughts on the matter?

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"Draw your sword with a heavy heart, but swing it with a heavy hand"

That's how we played it. Rules didn't prohibit it, so it was allowed. Just wanted to make sure we did it right. Thanks