@Crystal5, No apologies needed! Let's see if I can answer your questions one by one.
1. Changing the nightly stay after a guest has made a reservation request: All hosts should have their calendars updated with the price they want to charge. Matter of fact, in the near future when the new Terms of Service go into effect, it will be a violation of the T of S to ask for more money than what the calendar shows. That said, there are some circumstances where a host may want to charge for a special service or fee (pet cleaning fee, for example). In that case, if you and the guest are in agreement, tell the guest to cancel their request (only if you have not accepted the reservation) and then send you an inquiry. When they send you an inquiry, you can then send them a Special Offer for the amount you wish to charge. Or, if you and the guest decide that another listing would suit them better, again ask them to cancel their request and send an inquiry on the correct listing.
Bottom line, though, it is your responsibility to keep your pricing and available dates updated on your calendar.
2. Minimum and maximum night stay: You can set this in your Availability Settings which can be located to the right of your calendar. I have set a minimum of 2 nights and a maximum of 21. There are other options in that section which you may find helpful.
3. Charging for Taxes: If Airbnb isn't automatically collecting taxes for you, you will have to do it yourself. Airbnb doesn't collect for me either and, yes, it's a real pain. In reading the new Terms of Service, I noted a section called Opt In. I believe Airbnb is working on a way for hosts to opt in for collecting taxes, but, alas, it's not here yet. In the meantime, you should check with your local taxing agency to find out how much you need to collect and set it aside for remittance. I have to pay 10% in Occupancy Tax and have to pay and remit on a monthly basis. It's extra bookkeeping, for sure. But I do keep track of it as I deduct these payments in my Federal Tax return.
Hopefully this helps, if not ask more questions!