A few very quick thoughts. Wide sweep smooth 45's & 90's will be much better than tighter radius or segmented. Two 30's would be better than two 45's and two 15's better than two 30's. An oversized end cap (eg, 8" to 10" reducer + 10" cap) will be better than a typical 8" cap. IOW, almost anything that can reduce the static pressure will help. 10" duct would help as well but might also result in too low of air velocity. MUA is Make Up Air. If you're sucking 600 CFM (with your setup about 100 CFM is more likely) out of your flat then you need to bring that same amount back in. In your case it is doubly important as too little MUA, as @kaseki mentioned, will cause your hood blower to have to work much harder against the vacuum that will be created in your house and so it's performance will go down even more. You need to do everything possible to help your hood do it's thing. You need to talk to a competent HVAC person.

The condensate collects inside the air handler where the evaporator coils are, you should have seen the pan and any tubing. There must be a pump that takes the water outside? The only connection between the outdoor coils and the indoor air handler are the refrigerant lines, and they are sealed, with liquid/gas refrigerant inside. Nothing for you to smell there unless there is a leak, and then your AC would not be working. I’m guessing as to the configuration of your system, outside condensing unit (heat pump?) and indoor air handler with duct work that goes to various rooms. Using my imagination, I have no idea what you actually have there. How did the AC guy clean the coils? Did he use some kind of chemical that you might be smelling? Maybe close to the garage wall? Hosed it off? If he used a chemical, can you ask to smell it? Does the upstairs system that you don’t use have it’s own condensing unit and air handler? I’m not an expert on this, I just used to work for an HVAC company many years ago, not as a technician though. Maybe someone with another idea will chime in.

here are a couple of previous threads with some ideas... https://www.gardenweb.com/discussions/5448833/need-help-cleaning-downdraft-cooktop#n=6 https://www.gardenweb.com/discussions/3945734/tips-for-cleaning-jenn-air-downdraft-vent#n=7

Might kind of depend... How old is the place? The air system- as in the furnace/filters and AC if you got one. Do you have any reasonable expectation that the previous owners cleaned the ducts at all? And how long has it been since the system has been run- if it's been shut off prior to your getting your new home. Do you think you might want to paint the place, or it needs freshing up? Does anyone have allergies to cats or dogs? Duct cleaning is often something overlooked. If there's smells of anything, cleaning whatever dust/fur that might be built up in the ducts can help. If you tend not to run central air much, or it hasn't been on.. might have been a while since the furnace has been fired up, or the furnace heat might make otherwise less noticeable duct smells come out. If you want to paint/freshen up, it's better to have the ducts cleaned and your air system checked/filters replaced before doing so. If you are putting effort like scrubbing floors, washing carpets, and cleaning walls and ceilings into the cleaning prior to moving- then yep, getting the ducts cleaned too is worth it IMO.