Spiegel wouldn't lie to you - he's telling the truth. Like JaniceNL said, the issue is tension. The main problem you're dealing with is that fixing the m-shape means removing sections of skin and pulling and stitching them laterally (tension from side to side). If the doctor tries to remove too much skin, ie correct a very strong m-shape in one surgery, the skin will be too taut and the scar may separate, cause hypertrophic scarring, and/or permanent hair loss from shock. However, Spiegel would likely not do that - he'd do a good job fixing the m-shape. I got mine done by Dr. Rossi, invisible trichophytic incision but luckily I only had a moderate m-shape. Since I was only fixing the m-shape, he just did a behind the hairline coronal trichophytic incision in which the hair is already growing through. Dr. Spiegel does that as well. So I say go for it, your doctor knows what he is talking about.
However, if you do that you cannot move your hairline down much at the same time because the tension would be vertical and lateral. See Dr. Ousterhout's book on FFS - the tension in both directions would work against one another. For this reason, Ousterhout suggests that these be done in different procedures with a minimum of 6 months in between.
In my case, I wasn't satisfied with only fixing the corners. My hairline was too far back. Since I was in Buenos Aires already, I was able to get 1400 hair grafts (4000 hairs total because they were mostly grafts of 3 except the front most transplants). It's only been two months but they're growing in beautifully. The advantage of using hair transplants is that 1) they are far less expensive in other countries - I paid $1600 for mine, 2) the doctor was able to create exactly the hairline I wanted - he drew it literally with a pen until I was satisfied and that's exactly what I got, and 3) I have no scar on my forehead but it was lowered a centimeter in the middle and sides and an inch in the corners. I will update my thread (Rossi - June 3rd) with a picture of it. The only downside of doing it the way I did - coronal incision, fixed m-shape, and transplants - that there are still some wrinkles on my forehead from not being pulled tight with a surgical hairline lowering. Also brow lifts are more effective with surgical hairline lowering and are less likely to far more than expected. My fell more than I would have like personally.
There's as detailed an answer I can give. Spiegel is a great doctor, even if some don't like his attitude. That's what happens; some people get really good at what they do and take too much pride. I've heard people around here say that but I wouldn't care because you're in such good hands though! So happy for you

You're going to be even more beautiful. Good luck ShadowCharms!
- Ruby