I'll try to answer your questions paragraph by paragraph.
1.
A receiver will generally offer many more features than an integrated amplifier. Here are some of the main things a receiver has that an integrated amp doesn't: video switching, at least 5 channels of amplification, multiple digital inputs (coax and optical), built in analog-digital and digital-analog converters, surround sound decoders (DD, DTS), and AM/FM tuner.
The A300 only has a 2 channel amplifier, so even if you bought a DVD player with a built-in decoder, you'd only have enough amplifiers for 2 channels. You'd have to buy more amps to drive the other channels.
2.
Did you mean amplifier, or integrated amplifier? A separate amp has only one input per channel, and is almost always used in conjunction with a pre-amp or receiver. So you'd plug all your devices into the pre-amp or receiver, and then plug your speakers into the amp.
3.
As mentioned above, yes, you CAN buy an amp AND a receiver and use them together. As long as the receiver has a pre-amp out feature (most do these days), you can connect a separate amplifier or multiple amplifiers directly to the receiver and bypass its built-in amps.
4.
Yes, you CAN use a receiver for home theater. Almost everyone does this as it's the easiest and most strightforward way to do it. If you choose to go with a 2.1 setup, you can have your TV connected to it by piping sound from it into one of the inputs on your integrated amp.
I'm of the opinion that you should go with a receiver, as it gives you many more options for your money.
