Scenario: A star system with multiple planets, in particular a habitable planet which has no axial tilt such as this, and the next planet out being a gas giant.
The gas giant is ignited using a method which does not change its mass such as Thermonuclear ignition.
the orbit of the gas giant is close enough to the orbit of the habitable planet/big enough that, at the closest the two planets can get to each other, the ignited gas giant will increase the average global temperature by 7 degrees Celsius, similar to the difference of Summer and Winter.
The two planets have different years, so that the habitable planet will cool down and heat back up depending on its proximity to the new sun, creating something like seasons (which it did not initially have due to no axial tilt).
1- is this possible?
2- what effect would the intial ignition have on the habitable planet, I expect the main effects would be to weather and the life already evolved, but I would appreciate hearing about other effects as well if you can think of them.
3- What are the longterm effects on the habitable planet, in particular what would happen as the gas giant moved away from, and back towards the habitable planets.
EDIT: reworded to, I hope, be clearer.