I'd recommend just throwing out a direct question to whomever they are having you deal with. I'd also recommend shopping around for the best possible scenario for you. As long as it's not costing you anymore in the long run, go w/ the builder's lending program, but do your homework. It could save you in the long run.
Just a note on your final paragraph; the reason they won't let you bring in a realtor now is because they don't want to pay one. Generally a realtor needs to register their buyer prior to seeing the builder. It costs the builder money to deal with a realtor, but they generally like to maintain good relationships w/ realtors, because we drive a ton of business their way.
I hope this is helpful! Feel free to contact me w/ any additional questions.
Also, they are VERY eager to work with unrepresented buyers as they are able to make more of a profit on you! And the fact that you don't have someone behind you watching your back... even better!
Good luck to you in your pursuit! If I can be of any further information, please contact me!
Art Hotes ~ RE/MAX Classic
Certified Negotiation Expert, e-PRO REALTOR
Using builder financing is highly profitable to the builder and is not always in your best interests. If you are able to recieve a percentage off of the house in lieu of the financing, you may be better off. You should do a comparison between the special rate and market rates and the difference in price to see which is better.
Sometimes it works out better to take a lower rate and higher purchase price which is why the builders offer the special financing. (then they can sell you a more expensive home) I would ask for a discount if it were me.