I'm still a bit hazy on the reason for the fold-away warp engines.
You mention something about power transfers from the engines when folded in? Are your warp engines not powered from the central warp core/engine room?
Well the primary reason for folding nacelles is to protect them during combat, making her excellent for blockade running, or hit-and-run missions where the ability to leave even after taking heavy fire is essential. The reduced profile of the ship with nacelles folded in means a smaller surface area for the shields to cover. Reduced shield power requirements means more available shield power.
The ship is also supposed to have a new plasma exchange system that utilizes warp plasma in the nacelles as an auxiliary power source through a larger network of EPS conduits (which are already present in Federation ships, and already use small amounts of warp plasma to help power other systems). With the nacelles in, Durandal has nearly three times the available EPS exchange stations, instead of the few available through the warp pylons on most starships. The increased number of EPS exchanges between the warp nacelles and secondary hull allow both more warp plasma to be siphoned off for auxiliary power, as well as a much more expedient return of that plasma to the nacelles when preparing for warp. Starfleet protocol dictates at least 60% of warp plasma remain in the nacelles in standard conditions, allowing a maximum emergency warp speed of 4.5 in Durandal, increasing as warp plasma volume in the nacelles increases. Approximately 74% of the maximum amount of warp plasma siphoned off is returned to the nacelles via those EPS conduits in 3.4 seconds before the nacelles begin extending. The remaining 26% is returned to the nacelles via standard EPS conduits in the nacelle pylons, carefully timed to finish .14 seconds before the nacelles are fully extended. Nacelle extension takes 2.67 seconds.
Durandal's warp nacelles are available for full operation when retracted into the hull, with the option to extend them while in warp. In the retracted position, engaged warp nacelles create a much less stable warp field, and have also been shown to cause interference with systems in the adjoining section of the secondary hull, as well as potentialy negative impacts on the health of crew members. For Durandal, maximum suggested warp operation with nacelles retracted is 5 minutes (after this point system difficulties and heath risks become an "unacceptable risk", as they increase with exposure), while the ships systems have a built in 7 minute automatic extension, which can only be overridden by the chief engineer, captain, or first officer of the vessel.
I know, the numbers are pretty much made up on the spot, but EPS conduits are already well-established canon.
If you don't feel like wading through technobabble, suffice it to say that the plasma can be used to help power the ship. They need the plasma for warp though, so they're not allowed to take it all out of the engines (usually). The ship is designed to return all the plasma to the engines before they are in position to go to warp. It takes about 6 seconds for Durandal to be able to go to full warp from impulse. They can also go to warp with the engines in if there's an emergency, though its dangerous.