Yes, HALion can handle 16 tracks per instance so you’d need to split them across multiple instances if you need 17.
To the best of my knowledge (emphasis because it’s not my area of expertise), Dorico can’t automatically route the same track to multiple MIDI channels.
HALion can, of course, but endpoint configurations are (as I understand it) a one-to-one relationship.
Hopefully someone will set me straight if I’ve got this wrong.
It strikes me that you seem to be going rather an odd way around achieving a pretty normal result. Your string players are solo players, which means that by default Dorico’s routing them to the HSO Solo instruments rather than the HSO Combi instruments. If you want HALion to use Combi instruments, because you want your strings to sound like an orchestral section rather than a string quintet, move your string instruments (in Setup mode) to Section players.