I don't know if my responses are in line with other hosts, but I don't think most of these items warrant any form of refund.
#1) Your wifi router got fried in the storm last night so there is no wifi and guests want partial refund!
$0 - my place is in the Caribbean, and interruptions to Internet service, or other utilities happen regularly. To manage guest expectations, I state on my listing that restoration of service may take a few days.
#2) Guests found hair in the shower and now want partial refund!
$0 - I will come clean the room again in the guest's presence, and apologize for any inconvenience.
#3) You accidently walked in on some guests not realizing they were already there. You apologize but you woke them up and they want a partial refund.
$0 - I think this situation may more likely occur when hosts share spaces with guests. My guestrooms are not on the same level where I reside. I would need a very legitimate reason to enter their room, and would have made sure to knock loudly on the door before entering.
#4) The parking spot that was reserved for guests has been taken. They have to park 10 minutes away and walk. They want a partial refund.
$0 - If it's not my car, I would attempt to locate the owner of the vehicle parked in the reserved spot, and have them move. Depending upon the impact to the guest (some have mobility problems), I may call to have the rogue vehicle towed. Reserved parking doesn't guarantee the prevention of an unauthorized vehicle parking in the space.
#5) That good old air matress finally gave out in the middle of the night. They want a partial refund for having to sleep on the floor.
$50 - Sleeping is a significant component of staying in a room. A complete refund is not offered, because the other amenities were available and functioned properly.
#6) Oh no the codo's pool is closed for cleaning today. Guests are disappointed. They want a partial refund.
$0 - Pools and hot tubs are routinely closed for maintenance even at the best resorts, and guests are offered only an apology. Manage guest expectations by informing them ahead of time that the condo routinely cleans the pool, and there may be a time during their stay that it will be unavailable.
#7) Guests locked themselves out of the only bathroom late at night. You have no idea how that happened. Now they want a partial refund.
$0 - There is a reason why bathrooms have special doorknobs. In the US, we call them privacy door knobs. Push a pin in the outer door knob, and open the door. You can also leave the pin with the guest.
#8) You left breakfast items for only 2 people when there were actually 6 guests on the reservation. They want a partial refund.
$X-amount it would have cost to provide the items for the four others, plus the grocery delivery fee. I am not willing to pay for all of them to have a prepared meal outside of the home.
#9) The refrigerator has a funky smell. Guests didn't want to use it. They want a partial refund instead.
$0 - I'd clean the refrigerator again, throw out the offending items, or offer to let the guest put their food in my refrigerator.
#10) Coffee machine is broken. They can't get it to work. They want a partial refund.
$0 - What is the situation? Is the machine is broken, or the guest just can figure out how it works?
#11) 4 guests were staying at your place. Sadly you only had 2 towells out and they couldn't find others. They want a partial refund.
$X - reimbursement to the guest for the additional towels, and the guest must leave the towels upon departure.
#12) Guests had asked you if your bed was hard or soft before arrival. You said soft. Whey they arrived they said it was too hard. They now want a partial refund.
$0 - Bed comfort is subjective. What I consider to be soft may be like sleeping on a board to another person. I would only provide the manufacturer's classification of the bed when it was purchased (soft, medium, medium firm, firm, extra firm, ect.).
#13) The air conditioner is broke. Guests were left with a hot room. They want a partial refund.
$10 - My rooms all have ceiling fans and windows with screens. Although this may not get the room temperature to the same level as an air conditioner, there are other cooling methods available.
#14) Your hairdryer has gone missing. You had it listed as an amenity and they now want a partial refund.
$X - reimbursement to the guest for a replacement hair dryer, and the guest must leave the hair dryer upon departure.
#15) Guest found a few bugs and a spider in your place. They want a partial refund.
$0 - My place is in the tropics.
#16) A pen was in your washing machine and it damaged a couple pairs of pants and some shirts. They want a partial refund.
$0 - It is the guest's responsibility to check the wash tub before putting in their clothes.
#17) Guests complain that your listing says a local attraction is just 10 minutes away but it turned out to be 30 minutes for them. They want a partial refund.
$0 - The estimation of travel duration is subjective. If journey times are very important the guests, they should check Google Maps, Bing Maps, Mapquest, Waze, etc., for more accurate route directions and travel time.
#18) Guests ran out of hot water. They are upset that not everyone can shower. They want a partial refund.
$0 - I will assume that the hot water heater is the appropriate size to accommodate the total number of people staying at the location, and running out of hot water is not a normal occurrence (otherwise, the host would have gotten a larger capacity water heating unit). If this is the case, I am not compensating the guests for being inconsiderate and using all of the hot water.
#19) Guest says your place gave them allergies in the eye. They will have to go to a doctor for a perscription. They want a partial refund.
$0 - I would have to review the medical report, and it would need to document all previously known allergies or prior allergic reactions. Guests with a history of allergies or sensitivities, have a responsibility for their own wellbeing to make an inquiry about use or presence of triggering agents prior to/soon after booking. I use common household ingredients that are known to cause allergic reactions for some (e.g., prepare meals with nuts, wheat, onions and seafood; clean with bleaching agents, and use shampoos, soaps and laundry detergent with perfumes), and would take measures to restrict using them during the stay of a potentially affected guest.
#20) Your neighbor's dog was barking and kept guests awake and they want a partial refund!
$0 - Guests stay in private residences for the homelike experience. This sometimes includes enduring inconveniences caused by neighbors, their pets and children.