Yep, most likely.
Yes, any standard set of headphones should plug into a portable cassette player.
Any standard 3.5mm headphones will work just fine with a portable deck.
Any in ear headphones are the most portable and give great sound.
You will need to purchase full sized headphones seperatley,portable CD players come with mini headphones.
Most portable DVD player come with a free carrying case, they are expensive items and need to be protected.
You can use any DVD player cleaning kit to clean your CD player, or just use isopropyl alcohol.
No. You should use the adapter specified for the DVD player. Using the wrong adapter can damage the DVD player and/or the adapter and/or cause a fire.
Yes the Philips DCP951/37 9-Inch Portable DVD Player with iPod Docking is a very nice option.
Anyone can buy portable DVD players online, it is not hard to find and also cheap to buy. You will find a huge range of quality portable DVD players at a cheap price on Amazon almost any day.
Yes, most sony portable DVD players have USB input jacks. You can find however, many different types and styles of portable DVD players, so be sure to check the specific model that you want to buy.
I dont think there are or will be any TV antennas available for portable DVD players. DVD players are meant to play DVDs and cannot be used as Television, so any question or doubt of antennas does not arise. As far as my knowledge goes this isn't possible.
A DVD player typically does not have any network ability, it is designed to play a DVD which is put in and out manually. With no network, there is no need for an airplane mode. Depending on the specific airline and country, you may or may not have to turn it off during take offs and landings.