I think the advantage of Backstad and Tarling is they represent legitimate back ups to the Team Pursuit team - they won't be racing the final unless there's injury, but by taking them as the TP back-up on a RR quota, you free up a space to take a 4th sprinter. Backstadt is also a legit TT threat.
As for the Men's road race, the best three guys GB have for the parcours are all in other events - Hayter and Pidcock are strong 'classics hill' riders, Vernon's a strong 'classics sprinter', depending how the race goes. The men's RR, however is going to be absolutely lit, and GB may legitimately decide not to burn too many matches there - The news that Geraint Thomas is doing a Giro tour double suggests he won't be in paris after two relatively near RR misses; it maybe also suggests they aren't going to push pidcock for the Tour GC either.
Personally, therefore, if it were feasible, I'd pop Pidcock in the Road Race only and MTB, Tarling in TT only and back up for track. On the distaff side, I'm surprised Zoe's not considering MTB, as it happens, but I'd defo draft her in for TT, if not RR. Given the strong 'hour' riders in the GB pursuit camp, they don't lack imho for TT riders. To mangle a metaphor, the GB women's road team is a little bit of a case of too many indians, not enough chiefs - i.e. lot of strong team riders, not all that many on form finishers...
Anyway, I'd do that with a mind to including 4 track sprinters for the women, with Katy Marchant concentrating perhaps on the Kierin.