I have whisper warm and we installed a timer switch for the fan portion so we dont have to remember to turn the fan off when we are leaving the bathroom. Works great. I was told it cant go inside the shower though so ours is right outside where you step out and the heater is great. I too bought one bigger than the space called for and have no regrets. I read that some complained that the heater didnt warm up enough and that made me get the larger size. The heater is great! Remember though that it takes a whole circuit breaker of its own-20 amp if I recall correctly so make sure you have enough space on your electric box.

Yes, you can wire them separately. I advise putting the fan on a timer and the light on a different switch. The timer I used was Leviton 611-6230M that has presets for 5, 10, 15, and 30 seconds. We use the short times to clear out bad smells and the 30 minutes during/after a shower. You can get other models with different preset times. Sorry, I don't recall which ventilation fan models we bought, but I am very happy with them.

Usually the grill that covers the fan is on spring clips that will snap the grill up to the ceiling hiding any unevenness in the fan level. Is yours not like that?

Not a pro but when I did a gut remodel the HVAC guy worked on my kitchen hood which required moving the opening to the duct. My friends just did a HUGE HVAC renovation on their home which involved moving the actual unit from inside the house to free up space for a larger bathroom - digging under the crawl space etc. They did have an architect draw up the plans because it was part of a larger remodel but it was an HVAC company that actually did all of the work. At the very least call an HVAC company or several and see if this falls within the scope of work they do. And obviously you would want to make sure they are competent and reliable in this field and not just claiming expertise where they have none ETA - In my experience which is limited a GC is used when there are multiple aspects of a job to coordinate with different trades but when there is a discrete job you hire the skilled person directly. For example, I replaced my AC unit a few months ago and although I had originally "found" the guy because he was hired by my GC in connection with my remodel, I contacted him and hired him directly to install the new unit. Similarly when I replaced kitchen counters years ago I just used a counter company and didn't hire them through a GC because there was nothing to coordinate and nothing to be gained.