There are several long lasting and dependable 9v batteries available. Duracell Coppertops are a good choice as well as Ray-O-Vac and Energizer Industrial.
no
A car battery has six cells each producing two volts. A 9v alkaline battery has six cells each producing 1.5 volts. There are some obvious differences, like that a car battery can put out hundreds of amps and a 9v alkaline can only put out hundreds of milliamps, a 9v battery weighs 45 grams versus 45 pounds for a car battery, and so on.
the best way is to replace with a known good battery also a voltmeter - read battery under load - it should be near 9 volts
The nine volt battery is available in so many brands that it is hard to keep up anymore. I would think that the best one would be a charged one and go from there. I have never had a problem with any nine volt I've purchased, mainly because it is already one of the most expensive batteries sold.
you better not. you need a 9V regulator for this, not a resistor!
No.
no
You can buy 9V batteries from almost any supermarket
"Stronger" isn't a good word here. Given the most common D and 9V batteries, it's more a question of "different" than "stronger". The 9V has a higher voltage, while the D cell has a bigger capacity. What'll be best depends on what you need. It's like trying to decide if a sports car is better than a tractor.
very
use inverter
9v! More voltage, more strength! Many 9v batteries merely have 1.5Volt cells jammed together to make 9V!