Here is where i get real with you all - since December 6, [less than a month!] we have spent over $2000.. [not including $1400 we spent just in her 1st year of life alone!] My husband and i cannot take food out of our children's mouths or go into debt to pay for extreme measures. So given that, our course of action is to keep her on steroids as the Doctor sees fit.... what will determine if she has lymphoma vs IBD is that the steroids will only work for so long... it generally will only manage the disease [lymphoma] for a couple months and then she will likely decline.. and then it will be about making her comfortable.
IF she doesn't decline, and/or if she gains weight and muscle and returns to more of her normal self then it is likely IBD and will likely be on a maintenance dose of steroids to manage.
I think this is a good plan. Sad as everything may seem. But given all you've been told, the 'catch22' is in play now and you may never know. I would be going the same route as you and hope that it is IBD. My gut is leaning towards IBD, even without medical knowledge. I'm not just being hopeful, but I'm thinking of her age and also the hopeful words of
@Zippo and others. I would certainly be following this plan and hope and pray for the best.
My second Dobie was diagnosed with Leukemia when she was about 9 years old. I went the same route as you, going to a specialist, who though expensive, was very thorough, making me at least feel I understood what was going on with her and what my choices were in treatment. Like you, I was given a similar plan of 'if she doesn't get better, then we know what we are dealing with'.
Chemo was over $6000.00 and that was almost 10 years ago. And the one thing I took away from that visit was the fact that all that expensive Chemo would do was prolong her life a bit. He said they don't treat animals like they do humans. Humans get hit
hard with Chemo and can be very sick, but of course, its a human life they are trying to save. And they have other means to treat the side effects. With animals, they don't want to sicken them so much that their quality of life is worse. So the Chemo is seldom a cure. It's available to you if you want to make that choice, but I thinks its important to know that in the end, it may not save your dogs life.

Just words to the wise....
But back to your girl, I think the route you are taking is a good one. I look forward to your updates and will pray for her and your family to remain strong!