Provided I read the plants right I'd say both high light and N-stress can turn red plants even redder.
Low NO3 though is much more effective than high light. But it's the combo what you want to focus on in terms of
maximum red coloration.

IME Arcuata needs more than 2 watts per gallon and low NO3 levels (1-2ppm) to turn an intense red. The downside is if you let NO3
drop too low or let it bottom out for too long a time you'll easily stunt the plant. Daily fertilization with very small NO3 amounts AND a
higher fish loading for continous NH4 supply seems to be the best long-term approach. For short term purposes I suggest keeping the
plant at higher NO3 levels and to stop adding NO3 for some days like Tom did.

Now here is some speculation on the bending issue:

Arcuata tends to grow more upright
1. if it senses strong light from above
2. if fertilizer amounts are kept very stable (varying nutrients -probably changes in temps and other parameters as well- produces more side shoots)
3. if light with kelvin ratings below 10.000K is used


Best regards,
Detlef