I had the exact same problem.
Trying to book a LBV => LAS for two in J with a stopover in EWR and was given the price of a mutli-city itinerary (2x 130,000 + 50,000) = 310,000.
The attitude of the agents was just unbelievable, always polite but incredibly rigid. At some point I really though they were just plainly not understanding me.

An agent told even told me stopovers were not allowed.
Another one told me I already had 3 stopovers in my trip, hence the high mileage required (confusing stopover and connections again).
Most of them did not even know there was an award chart available on their website, they kept requesting a screenshot of a better "price"...

Well, after a lot of frustration and a lot of unsuccessful emails, I decided to book anyway and pay more than required. In the meantime, I also opened a complaint by email with Lifemiles and they are currently "looking at it"; I asked for the refund of the extra 50,000 miles + applicable taxes.

The last I heard though, was from a guy basically acknowleging that a stopover is indeed allowed but that the only way to book a stopover is to use the multi-city booking system which yields a high number of miles... a lot of nonsense.

An interesting point though, it is in fact possible to "buy" miles at ~16 USD/ 1000 miles at any time and an unlimited amount. Typically, when a ticket is cancelles, the cash amount is not refunded but instead, the full miles amount is refunded along with the taxes - 50 USD fee.

In short, I paid 124,000 miles + ~3000 USD + taxes and I could cancel and get a refund for 310,000 miles + taxes - 50 USD fee. This sounds too good to be true, unless the agent did not know what he was talking about.