As a clarification, what supports are we assuming are reasonable?

Not a proposition to move a character, but I was curious as to what supports are assumed to be viable in this game. I remember that HectorXEliwood was one, and I'm sure that Kent/Sain and Raven/Lucius were ones as well. In that case, are we assuming that all supports that take around 70 turns to an A for characters that join early (before or around chapter 16-17) are viable? assuming 75 turns is the cutoff (And this could be too high), would the following supports be viable?

HectorXEliwood (57 turns to A)
EliwoodXNinian (49 turns to A, although her availability is limited, so maybe not)
MarcusXLowen (75 turns to A)
RavenXLucius (71 turns to A)
RebeccaXWil (71 turns to A)
RavenXPriscilla (75 turns to A)
FlorinaXLyn (49 turns to A)
FlorinaXFiora (52 turns to A)
KentXSain (71 turns to A)

Or are some of these unlikely due to move gaps, etc?

I wouldn't list Hector x Matthew as very viable because they're very often off doing different stuff when both deployed.

You forgot to actually list LuciusXRaven.

HectorXMatthew probably has issues because of roles (tanking and seizing vs. thief duties?) and Matthew not being fielded in some chapters later on. Considerable doubt as to whether it's hitting A, and the C/A levels of the support give pretty shoddy boni. GuyXMatthew only takes 3 more turns, Guy doesn't need to seize, Guy has equal Move to Matthew post-promotion, and giving full-Attack instead of half-Attack and half-Defense makes it better at support levels that aren't B.

If we're willing to support the Archers together, ArcherXDart is viable too.

If we're using both Farina and one of her sisters they could easily get a C for a small but tangible boost, and then a late B.