Exhausting a sealed room?

Three questions. (Can’t find the answer using the search)

1) If you have a sealed room using CO2 with a PPM controller, air conditioning, and a dehumidifier do you have to exhaust the room? If so, why? And how often does this need to be done?
2) If you had only enough room to have half of your plants in the above mentioned sealed room would it be best to be in the sealed room the first 4 weeks of flower or the last 4 weeks of flower? (Assuming an 8-week flower cycle).
3) When in the flowering period is a plant going to require the most light? If you had 2 rooms 1 @ 11750 lumens per square foot and 1 @ 8000 lumens per square foot would it be better to start flowering (first 4 weeks) with the higher lumens or finish with the higher lumens (last 4 week).

1) If your temperatures are good, then no. You'll definitely want to maintain proper CO2 levels.
2) The entire flowering time would be good. Otherwise I suppose the last 4.
3) It'd depend on how well you treat the plant in flowering. If it maintains(and grows additional) foliage, probably the last 4, if it loses foliage(fairly common), probably the first 4 weeks.