Combustion.

At combustion, planet loses its own rays due to close proximity to sun.
A) It means that the combust planet obscured by the rays of the Sun, is considered impotent (weak to act independently).
Combust planet express itself through Sun.

Important Note : Retrograde planet does not lose its rays at combustion.

Now,
B) Sun is the Karak for lagna, signifies self (EGO).
Due to closeness of combust planet SUN is also affected with the qualities of the combust planet

To say to someone that Mars is combust and therefore weak (impotent) will not ring true because the person KNOWS that the traits of Mars are strong in their nature (self). Similarly, although Mercury suffers combustion, the qualities of Mercury are also reflected through Sun... The native KNOWS he's not lacking intelligence.

However, ego and pride (boastfulness) would likely be quite prominent in regards to intellect (Mercury) and force of character (Mars)..

C) Sun is karak for Dharma and Agni which purifies the individual beings by cleaning them with Spiritual heat.
To say,
Combustion of Jupiter will make you pass through a phase of spiritual growth like gold passing theorugh fire to come clean and combustion of Saturn will burn the sins attributed to adharma you did in the past birth by forcing you to serve elders and father.

D) Also look for natural friendship and enimity of Sun with combust planet.
For example,
Combustion of Venus indicate delays in marriage ( perhaps Sun shows it's anger) and it also indicate Dharmic attitude of Venus at relationship, no extra marital affairs or love affairs...

E) The combination of combust planets is also an important consideration.
For instance, the combined influence of Mer/Mars (on Sun) would be different to the combined influence of Mer/Ven or Mer/Jup...

F) How results will be manifested can be seen through house and nakshatra lordship yogas.

//3) What could be the effect of a combust and retro Mercury ?//

Retro mercury will not lose it rays so combustion results will be not pronoused and Budh Aditya yoga will give its full effects.