You don't need a "scientific consensus", you just need "math".
Consumerism (the exploitation and consumption of natural resources to support modern/unsustainable lifestyles) is what produces pollution and environmental/ecological degradation. The magnitude of the damage done (D) is thus the product of the lifestyle (L) and the number of people living it (P).
L * P = D
Thanks to "math" it also holds true that:
P * L = D
Neither P nor L are solely responsible for D — they both contribute. For the purposes of calculating D they are inseparable. Neither is "the biggest" because it is the product that is the problem. Thus no-one should be overly fixated on the issue of P or L. You can achieve a 10% reduction in D by reducing either by 10%.
The world is far too complicated and interconnected to ever support the simplistic notion that X — and only X — is the cause of Y. There are always multiple factors in every equation.
As any student of history knows, the inefficiencies of medieval agricultural practices (read: amount of land required to support a peasant family engaged in subsistence agriculture) means that it would simply be impossible to support 7.5 billion people on Earth using them, so scenario 2 is not viable at all — the farmland required would far exceed the arable area available.
Crunch the numbers yourself. Figure 30 acres of arable English
farming land per family of 5–6. Only 35% of the country was arable.
Snap productivity to a diminishing sine wave as you leave the ideal
temperate climate zone and head towards the sub-tropic or the sub-polar
zones. Integrate over the planet.
A modern lifestyle with 350 million people, on the other hand, is doable. I crunched the numbers back near the turn of the millennium (initially following energy flows via trophic layers) and 869 million people with a lifestyle equivalent to those in the USA during 1999 were possible. Allow for per-capita increases in energy consumption over the last 20 years and that number is now probably around the 600 million mark. Scenario 1 is totally sustainable.
A note: It seems that you may not be aware that, in order to live a medieval lifestyle, people needed to burn a lot of wood. The primary constraint to population growth (in temperate climates) was the availability of trees. Trees were the energy source that fuelled growth. That why, wherever a town or city existed, the land for miles and miles around would get increasingly stripped bare of trees that got burnt for wood. Once the trees were gone, then the springs dried up, and soil erosion kicked in, crop failures would increase in frequency and severity, and peasants in the urban areas would freeze to death in winter. It was only the 'miracle of coal' that allowed the population to grow above 600 million circa 1700. People dug up coal instead of cutting down forests. The forests returned, agriculture prospered once more, and populations boomed.
The reality is that the overwhelming majority of the current global population is addicted to a convenient, portable and highly-dense form of energy. It's addicted to oil. It cannot be sustained without oil. We currently burn 9 Joules of oil to produce every Joule of food. As the oil runs out the planet will be forced to ratchet down to less convenient, less portable, and less-dense forms of energy. That will impact the food supply. The population will track down accordingly.