So, with the updated information we have on the river: I think Norn is in much tougher position than some of us expected, since there are only 2 feasible natural crossing, plus a couple to small for an army to cross without a secured beachhead. The room for clever moves is much more limited, as Norn only really has 2 options: Take the bridge by constantly wearing us down, or pin us down long enough for another army to cross. Given the defensive advantage the battlefield brings, I am reasonably confident about leaving the 6th as an auxiliary force while we take care of attempted crossings. If we don't screw up, this is grueling and slow battle that will involve heavy losses due to the defensive advantage with our battles.
In terms of army expansion: I am actually somewhat considering getting one unit of siege artillery for now. It would be expensive influence-wise (leaving us with only 50 influence), but also immensely useful for destroying enemy pontoon bridges and their own fortifications. We are going to have several pitched defensive battles somewhat comparable to defending a fortified position, so now might actually be the right moment to invest in siege artillery. We will need to get one regiment of it eventually, so yeah. Even a fairly fresh regiment could destroy a bridge in 3 or 4 hits (160 hits to destroy a bridge tile) and human artillery learns fast. This would vastly increase the risk of any crossing as there is a risk we could demolish the bridge, leaving the assault forces stranded on our side. And it could soften enemy terrain for eventual counterattack, as we are able to negate the dug in troops. Also, this would massively complicate any attempts to build pontoon bridges, who are probably fairly fragile. Definitively something to consider, if we expect to win a couple more battles.
Aside from that, I also think we can put expanding our infantry corps off for a bit since I expect our frontlines to be really small. If we are planning to expand, an increase in our artillery corps is probably the most useful, since we are defending an area of limited size rather than full battlefield with open flanks to our sides. Though there is also the option of getting another infantry regiment if we burn through a bit of influence (55 for 1100 regular infantry troops in the old economy, so an regular elf regiment plus some reserves). Which isn't a bad investment, it's just mostly an issue of time. WIth a drill regime and us using influence for reinforcing for now, we could get up to a quite respectable 7 drill this way in the coming weeks (harsh drill + recruitment T1, harsh drill + founding T2; 4+4 -1 = 7). Being on par with a nornish provincial army would have it's advantages, especially if we aim deny them other crossing by moving alongside them. The resulting losses could be compensated for by using influence, which would work out fine with winning battles (+100 = 2 thousand regular troops, more than enough for army expansion). We wouldn't gain new influence for specializing our army further, but in return our army would be fast while marching (expetional by Arnése standards anyways) and able to expand more easily if we catch a break.