If they don't have a good plan now, chances are very slim at best they'll have a good plan further down the road, and even more slimmer to have an actual 2015 car ready for Bahrain. Maybe the 2014 chassis is adaptable and once done, acceptable as a 2015 chassis. Still requires a big investment.
However, they will receive the price money if they show up in Bahrain, in other words everybody will have to wait until a fifth into the season for their 4 million. Sounds all very useless to me. They should have distributed it across the 3 smallest teams atleast. 4 million isn't going to make any big difference.
I also want to underline I doubt Fernley is in anyway interested in a good plan for Marussia. As you self said, this is a sport of self interest, being it whether declining Ferrari another team or getting the 4 million.
At least with Bahrain, they have 10 weeks to get a 2015 car cobbled together & crashtested. Let's be honest, there's 9 other platforms out there now wth plenty of ideas to incorporate, and 10 weeks ago, the 2014 season was just finishing so theoretically no reason why they couldn't get it done.
Realistically of course, if they're not in a position to get a 2015 car knocked up in 10 weeks, they're really not in a position to be racing in 6 weeks, even with the old cars and as you say, equally likely they're going to be unable to satisfy the paddock of thrir credentials.
Personally as said though I'm in agreement with them that without a full and clear business plan and funding there is no way the group should get access to the prize money, and a place on the grid.
The issue is producing a car which normally takes close to 10 weeks on its own. Producing the molds and baking the carbon fibre layers is a very delicate process. Couple to that they need to find new suppliers (because the old ones will not deliver until the debt has been paid), and sufficient funding to pay the supplies up front because not one supplier will want to delivery without enough assurances, as in inmediately paid.
Also you can't just take a look at the competition and slap that on your car. You'd need cfd and windtunnel analysis, something that takes months to optimise. In short there are only 2 options: either producing the windtunnel model to the actual prototype, or a car to as close to the 2014 one as possible.
Producing a new car in this small timeframe is too unlikely. A customer chassis like Dalara is probably the only way, provided the chassis is similar enough to the 2014 chassis to fit all the 2014 parts. Producing new parts is too costly and takes too long. Either way, Dalara will also want to be paid before producing anything.
The best shot is adapting the 2014 chassis, but again that too is difficult. Racing the old 2014 chassis would have required to find suppliers to produce spare parts. That would have been challenging, but realistically a possibility.
But again, after more consideration, running a 2014 car between much faster 2015 ones is not save.