Use a hygrometer to determine relative humidity. Excessive humidity is normal in many basements in summer. Depends on the climate. However, too many people figure if they see a dehumidifier it means there's something wrong with the home. So I've learned to hide ours away when prospective buyers are taking the tour. Is there anything we should look for? I've bought dozens of homes. Signs of water penetration and hidden damage are the biggest concerns. You've already learned to use your nose! Structural integrity is the next thing. Look for unusual settling, cracks (including patched ones) in corners, and repairs--added support posts, for instance. If the exterior is barrier EIFS (synthetic stucco with no weeps), get a professional inspection. Otherwise, walk around and look carefully at the condition of the trim, drainage and settling. You should take a look in the attic--just pop your head in and shine a light to check for waterstains, indicating leaks on the underside of the decking. I've usually bought homes built from the 1880s to the mid 1950s. So I'm not looking for perfection, but for fatal defects. Here is a link that might be useful: Humidity Control

Keep in mind that a well-sealed house will have minimal humidification requirements compared to a leaky one. There is no significant difference between the source of heat, forced air, radiant in floor, radiant baseboard ,etc in the humidity except that these might affect air leakage (forced air with ducts outside of the house envelope). I expect that you will find that in the same geographic area, compared to an old, leaky home, you will find that on those real cold days is the only time you will want the humidifier. I don't think you can use the most common humidifiers in AC ducts when they are not used for heating. You are likely to get condensation running out of the ducts. I could be wrong, there might be a way to do it.

Sorry, but I think you're overthinking this &/or misinformed. I use pebble trays under many things, Holiday cacti, Hoyas, Spaths, tropicals I always water the plants to overflow so the water goes into the pebble tray, it needs that to be of any use. Been doing this for years, never clean out the pebble tray, don't use anything but clear water; never had any issues w/ it. I have too many plants to move them for watering. Everybody gets watered in place & over time, one gets the feel of how much water to water into each particular pot, for a reasonable amount of it to overflow onto the pebbles to then be re-absorbed back up to the plant as needed. (I don't use humidifiers.) You'll have to experiment & see how to make it work for you. But w/ that few plants I can't see this being a big deal. I don't know or grow citrus, so that may be different.

Low humidity in the winter is usually caused by leakage issues. It is disappointing to hear your new construction is suffering from low humidity. Even a tight house can become dry when the outdoor temperature and humidity are low. How large is your house, and what size is the furnace? Does the furnace cycle on and off for short period of times? If it does, then the furnace is not running long enough for the humidifier to raise the humidity. I think it is possible to wire your humidifier to turn on your furnace blower whenever the humidity is low. This would avoid spending another $2800 on a second humidifier. If you have an Aprilaire humidifier then it may be possible to install a Aprilaire 60 humidstat with blower activation. I have no experience with this set up, but it is worth investigating if you have a compatible humidstat and furnace.