I have made some progress on this issue. In Jetpack's comments.php file, I commented out the following markup before the comments iframe:
<div id="commentform" class="comment-form">
I also commented out the following markup just after the iframe:
</div>
The page loaded fine in Chrome on iOS 7 and Safari on iOS 7 after making this change. To further test, I uncommented the above two lines of markup. The crash reappeared.
Next, I modified the Decode theme's /style.css file. I changed all instances of the .comment-form class to .comment-florm, excluding any .comment-form-xxxx classes, which I left as-is. After refreshing caches, the page again worked with Jetpack comments enabled and the Decode theme in Chrome and Safari on iOS 7.
I suspect that there is some sort of CSS conflict between Jetpack and the comment-form class that the Decode theme's stylesheet is using. I note that there are several uses of -webkit in the Decode comment-form classes. Perhaps one of those is the issue.
At some point, I will comment out the -webkits one-by-one and see if I can, by process of elimination, determine where the problem lies.
I have cross-posted this on the Jetpack support issue in case it helps the developers or other users of both Decode and Jetpack.