Have you confirmed that it's actually cheaper to rent in the city? I've found that I can often save money by picking up (and dropping off) at the airport where lower base rates counteract the additional surcharges. When in doubt shop around.
That's not always the case, especially for common car types. I've been looking for a car before, and often the best rates were at the airport, and so low that it more than made up for any airport fees. It does depend on the airport though since many have different fees.
It really depends on the timing. I've found times where there was so much inventory available at an airport that the base rate was as low as $8 a day. However, airports may have crazy demand some days where the base rate can be $80 a day for the same class.
That's been my experience as well. The only way to actually know if you're really saving money is to shop around. Maximize your leverage as a customer by being open to a range of dates, times, locations, and service levels. The more specific you get with your needs the less leverage you have to find them at a good value. I've read a lot of advice articles with overly specific rules that on further inspection turned out to be irrelevant or counterproductive for many of the trips I've taken.
You probably can, but then you'll need to factor in how the drop-off fee works. Sometimes it's a flat fee, and at other times it's rolled into the base rate. On top of that, you'll probably be paying the various airport fees. If you can get a better rate, then getting to downtown Portland is pretty easy.
