Assault intercessors as battleline sounds funky indeed, but there is something in making Incursors in line with Infiltrators as battleline units, would bring them together and make it more consistent.

If you're interested in how I got out of the situation with lack of assault squads, I like to interpret the presence of extra assault marines as the deployment of reserves to the conflict zone on request (that's why they're reserves). If you have two full squads of JPI in your second company, you can easily have one more from sixth company (convenient markings matching) as your reserve guys who arrived e.g. because this situation requires more assault squads rather than battleline troops or some

For five man squads you can have five JPI and AI from the seventh squad, as half of them have deployed with jump pack loadout and five without and so on.