Doubt on the Voltage Regulator Application with LiPo Battery
Your calculations are absolutely right.

Given your restrictions and goals: YES you could use several cascaded linear regulators (with different output voltages, of course, and respecting the required dropout voltages) and/or series resistors, in order to distribute the heat dissipation across several components.

However, a heatsink looks like a far simpler and better solution. Let's say you want to derate the maximum junction temperature to 80% while being able to operate the regulator up to 50 C ambient temperature.

That would mean a maximum junction temperature rise of 70 C. At 3.5 W dissipation, you'll need a 20 C/W junction-to-ambient thermal resistance. The TO-220 junction-to-case thermal resistance is just 5 C/W, so you would need a heatsink providing a 15 C/W case-to-ambient thermal resistance. That can be perfectly achieved with a passive heatsink (which could even by realised by bolting the regulator to your case/chassis if available).