Re the trident vs 727 I understood that Boeing did visit Dh but I doubt that the whole concept cme from their. More like the design parameters of the US majors which i think were on the lines of Dallas out of LGA or DCA leading to the extensive slat/flap system on the 727.
Also in general terms if you were going to scale down a plane from 707/DC8 size for short haul medium haul you would probably choose three engines as two were probably considered a bit risky or just didnt have the grunt and if you chose three engines you get one in the tail thus hight tail plane and therefore two aircraft that look alike.
Same with the IL62 VC2 need along hauiler got to have four engines . VC 10s needed a clean wing for hot and high empire routes , some of which visited basic places witha lot of FOD potential . IL62 long range , and Russias so big that can mean domestic and that can mean some rough airfields. either way no good having pod mounted engines on the wings to suck up rubbish so stick them on the tail in two pairs. Tail engines high tail so VC10and IL62 look a lot alike. DC9-1-11 etc so I think its the mission that makes the plane not copycatitng