Yeah, I'm seeing the same issue. The HTML is all there if you "show original". It appears to be a display issue. I'm told Google is working on a bug fix.
Temporarily, I'm just copying the html and sticking it in http://htmledit.squarefree.com so I can read it.
Until Google figures a way to have Gmail read your brain waves, it cannot reliably tell if you've read the message. Some mail clients mark a message as "read" only after it has been displayed a certain number of seconds, which might be closer to what you want. But that's not available in Gmail.
That's not to say that there aren't workarounds.
You could enable the Message Sneak Peek lab feature: Click the gear icon -> Settings -> Labs, scroll down to Message Sneak Peek, and click Enable. Go back to your inbox and right-click on any message. The message will be displayed in a popup window, without being marked as read.
Another thing I find myself doing, is mark a message unread after I've opened it, by clicking More -> Mark as unread (or press Shift+U if you have keyboard shortcuts enabled).
Also, starring messages is perhaps the most obvious way of keeping track of important emails that I need to deal with later. I use this in combination with the Multiple inboxes lab feature, so that a section of my inbox view is reserved for starred messages. When I've dealt with the message, I unstar it, and it is archived.
Best Answer
Some of this will depend on your personal settings. Assuming yours are close to mine, here's one way to do what you want to do.
Click the button that looks like an ellipsis at the bottom of your reply pane.
(If you hover your mouse it will tell you that it will "Show trimmed content")
This will expand the quoted text for you. Quoted text is indicated by a line on the left margin
Place your cursor in the spot where you want to break up the quote and hit Enter twice. This will form a break in the quoted text. Add whatever part of your reply you need to here.
If, for some reason, part of the reply becomes "unquoted", you can re-apply the quote format by selecting the text you want to change and using the "Quote" tool under "Formatting options" (or Ctrl-Shift-9).