Hi kaitodaimon, I'll answer your questions as best as I can:
1) Do you have any knowledge regarding the intro cutscene in stunt rally, since the intro cutscenes from the retro-LEGO games, are more detailed than the moden LEGO game´s cutscenes, in terms of lightning and overall details.
As others have pointed out, the intro cutscene is pre-rendered and in a different style from the modern games. The intro and victory videos were indeed done by another studio, while the in-car camera shots were made by IG. As for the increased amount of detail compared to the modern games, my guess is that back then LEGO didn't have their preferred style nailed down for cutscenes. I'm pretty sure that by now they will have very strict guidelines as to how minifigs need to look and move in animations (in whatever medium).
2) Do you remember how many of the models in the game were based upon actual LEGO sets?
As for vehicles, only the cars of the playable characters were based on existing LEGO sets, which were being developed at the same time. The characters themselves were designed by a few of IG's artists.
3) What was your biggest reference in the development of the models and the structure of the layout for the game?
For the cars, we received the actual toys, photos, and rendered images that were being made for the cutscenes (I will post some of those later). We didn't reference anything for the structure and layout of the game.
4) Is there any particular reason that none of textures for the minifigures had gradients implemented to them?
It sounds like you are using "gradients" to describe the rendering style. Some of the plastic in the pre-rendered movies in LSR looks more shiny/glossy than IG's renders. This has nothing to do with the textures, but rather a setting in the material. That setting in the material says how shiny/glossy should look when rendered. So the different teams used different settings for the plastic materials. The lighting will also be different - in IG's movies/renders, we aimed to have very soft shadows.
5) Were there any restrictions in the development process from LEGO that you remember?
Absolutely. Working with any publisher or holder of an Intellectual Property (IP) (also often referred to as a "a license"), there are always guidelines on what you can and can't do with the characters/world. etc. One starting point was that we were given was a fairly strict set of colors to use. So, for example, if something is red it needed to be a precise RGB value set by LEGO, we couldn't just pick a color red that we liked. This kind of stuff is standard when working with an IP.
Things got a little weird at times though. For example, at one point we were told to change the color of all the city roads from dark grey to light grey. We had based our roads on existing (but slightly less common) dark grey road plates, and we argued against the light grey because a) it didn't look as nice (due to the lower contrast) and b) it would be a considerable task to change it all (re-work all the road textures and re-render all the icons (twice actually because the game has two UI sizes)). I've seen some screenshots with the dark grey roads online, but they are all light grey in the final product.
One major change that LEGO demanded was the removal of the steering. People on this forum have discovered the hidden FREEFORM driving mode, which was actually the default mode during much of the development. The team was pretty gutted when that request came, but LEGO's point was that the game was too difficult for its target audience. Driving controls with an isometric view is quite counter-intuitive, so you can see their point. With the simplified controls, the game became a scalectrix type game. For the very first level of the campaign, we aimed to make it so easy that you could win by just holding the accelerator the entire race (with any car!). Obviously, that is the most boring level to test, so we had an orange do it. We placed an orange on the up cursor button on the keyboard and just waited for the result. The first level needed to pass "the orange test", which became a bit of a meme inside the team. I may have an image of the orange in question, I'll see if I can find it.