1. Generally speaking, I would recommend for each country you're thinking of visiting, to check what your cell phone carrier's charges are for roaming there, and if they let you use your domestic data and minutes, etc., and then take a look at the costs to get a sim card with a pay as you go package at your destination.
For some places, like Britain, it's super cheap and easy to buy a basic "burner" phone and SIM.
For an American visiting Canada, best bet is usually to get a Canada add-on to your package since Canadian cell phone plans are expensive.
2. As others have noted, most accommodation sites have category ratings in addition to an overall rating, and the ratings and comments for location will often give you a good indicator. My personal rule of thumb is minimum 8.0, though I may make exceptions, such as where the score is low simply because it's far from city centre, and/or in a primarily residential area but is close to a transit hub and other things I may want, like a supermarket.
3. Yes but YMMV. To use the Pacific Northwest for example...
*Portland, OR: I wouldn't bother as there's several blocks of cheap hotels right by the Parkrose MAX (light rail) station
*Seattle, WA: Green Tortoise is meh, but gets points for being right by Pike Place, and with a Target nearby. I would rather spend a bit more for AirBNB. Unfortunately hotels downtown are very expensive. You might get lucky and get a hotel for about $100 / night through a discounter.
*Vancouver, BC: Like Seattle but worse. AirBNB might be best bet.
**A rare example where I would intentionally rate a hostel over a hotel would be Lake Louise, AB, where I would rate the HI Lake Louise over its neighbour the Post Hotel. It's a way cooler property and cheaper.
I concur with
@Au Naturel that when in GB and Ireland, B&Bs are a cultural delight and the hosts are often a great resource who are happy to help you with whatever questions you may have. Family run hotels in Europe are somewhat similar as well in that they'll try to do their best for their guests.
4. It really depends on how long you're staying and any other places you're going to. In general, $100-200 in local currency, with some smaller bills and if possible coins, is helpful. You don't want to be at a train station trying to buy a subway ticket from a machine with a $50.
I avoid carrying more than $100 or so in my wallet, and in some places, like Italy, I would only carry a "dummy" wallet with €30 or so and keep a card and some €20s in a neck or waist pouch (lots of varieties available online, including metal free ones to avoid metal detector issues) and if I need to "reload" the dummy wallet, do so in a restroom.
ATMs are hit and miss. I recall being shocked that an ATM I used in Slovakia only dispensed €100 notes, since even €10 notes were heavily scrutinized at the train station. Needless to say that wasn't helpful and it was a good thing I had some change. Some ATMs will usually have a sign indicating what denominations are available. Weirdest experience was in Switzerland where I encountered an ATM that only dispensed 200 CHF notes.
When withdrawing from an ATM, you will sometimes be offered the choice of withdrawing in the local currency, or withdrawing in your native currency. Always choose to withdraw in the local currency since choosing to withdraw in your native currency will result in the machine doing so at its own exchange rate, which is generally at a very marked up rate (e.g. 10%)
As others have noted, enough to ensure you're okay. If you're thinking about buying stuff from street markets, or small boutiques that might offer a cash discount, then maybe you might want a bit more.
Most major stores quite readily accept the two major cards (Mastercard/Maestro and Visa). American Express (even within the US) is a bit hit and miss, but generally accepted at larger stores.
5. I always pack lightly.
6. As
@Misery noted, at the hotel and in the safe. Only bring what you actually need. Also, some places you visit may have airport-style security screening, in which case it's best to be carrying as little as possible.
7. It depends. Most tour groups have designated free time each day (or on occasion, a full "free day"), and there is no leaving and staying behind, or opting out unless you are abandoning the tour. Sometimes they will take you to certain places because they're getting a fee and/or kickbacks from a business or association to bring people there, especially for outlet malls, so they don't want you wandering off.
That being said there are a small number of tour companies that offer inter-city tours on a hop-on-hop-off basis, where there might be a five day trip, but it's officially five one-day segments, and at the destination city each night, you can choose to stay longer and re-join another day, space permitting.
