Question: When kendra [angle] and trikona [trine] lords are together classics say Raj Yoga is formed. But, does raj yoga give results if it formed in dusthana houses – like the 12th house?
Raja yoga gives results through the bhāva(s) [house(s)] in which it exists. It can give the results through the dusthana houses, or any other.
Question: Assume that the Sun as 7th lord is involved in Raj yoga with Ve and Mer. Does the fact that the Sun, being the 7th lord, is a mārakeśa (lord of a death-dealing house) cancel the yoga?
No it does not.
The 7th house is a kendra [angle], and the classics say the angle lords can form rāja-yoga. Why would they say this is the 7th lord cannot form the yoga???
Of the Kendra Lords, 10th lord forms the most powerful yoga, then the 1st, then the 4th, and the 7th is the least powerful.
Question: What if the planets forming the yoga are combust?
Combust planets can still form Raja-yoga, just be aware that their combustion may affect the potency of the yoga or altar the type of effect it has.
Question: What if the trikona and kendra lords all receive aspect from lagna lord Saturn
This makes those lords more significant, because the 1st lord will be able to express itself through them.