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As it's visible from the images below, when looking at the plane from the top, the engines point outwards and they are banana-shaped, why is that?
(unfortunately I couldn't find a real photo straight from the back of the airplane, from that angle it is way more noticeable),
https://upload.wikimedia.org/wikipedia/commons/9/93/De_Havilland_Comet.svg
The angling of the engines on later Comet models was to prevent engine-exhaust-impingement-caused damage to the aft-fuselage skin, which had been a problem with the Comet 1:
(f) The possibility of damage by jet efflux
During the operation of B.O.A.C. services, there had been some experience of small damage to the cabin skin, due to the buffeting by the efflux from the jet engines. This damage was partly in front of and partly behind the pressure dome of the cabin. As soon as it was observed, a systematic inspection was made of all Comets, and where any signs of cracking were detected a repair was made according to a scheme specially devised by de Havillands. Internal inspection showed that the buffeting was also causing slight loosening of the joint between the stringers and the skin in this region, and rivets were therefore inserted in order to ensure that this would not give rise to danger.
This point of possible weakness was under continuous observation. The steps taken to deal with it may be considered to be satisfactory, particularly since, where the repair had been carried out, no further trouble occurred.
It is, however, recognised by de Havillands that a situation in which it is known that such cracks are likely to occur is unsatisfactory, and among the improvements they intend to make on future Comets is one which they believe will reduce the cause of this damage, namely, a slight change in the direction of the jet pipes at their exits, with the object of diverting the jets away from the sides of the cabin.
[Report of the Court of Inquiry into the Accidents to Comet G–ALYP on 10th January, 1954 and Comet G-ALYY on 8th April, 1954, Part XIII, section (f), paragraphs 112-114 (on page 25 of the report itself and page 45 of the linked PDF of the report), my emphasis.]
The engines were mounted well inboard to reduce the hazard of asymmetric thrust.
Four turbines are placed so close to centerline of plane that even if two on one side cut out, pilot has little trouble maintaining straight, level flight. [photo caption, p. 101]
...
The Comet's engines are snugged in so closely to the fuselage that there's no need for the customary trim tab on the rudder. [p. 102]
-- Francis, Decon. "I Saw This Jet Liner Fly 500 m.p.h." Popular Science, 156(5), May 1950, pp. 98–104.
Protecting against that hazard also explains the deflection of the engines' exhausts, which places them closer to a line intersecting the airplane's center of mass.
Another reason for the deflection could be to prevent jet blast from overheating or otherwise damaging the fuselage's skin just aft of the exhaust. The dH Ghost turbojets were built a decade before the first high-bypass engines, so their hot exhaust had no surrounding cooler air.
A third, faint, reason might be to facilitate egress from the over-wing emergency exit windows, sliding aft and down instead of climbing forwards. But avoiding asymmetric thrust is the reason that is documented in the literature of the time. (By the way, that entire Pop Sci article is a hoot.)
