1) You can just delete it. I usually save on my computer for a while, just in case I want to move it back on the phone to re-flash, in emergencies, etc. I usually try to have either (but not both) a nandroid or the ROM zip on the phone, just in case.

2) The differences are aesthetics, features, performance. The thread for each specific ROM will usually have a good description of the features. But you may need to learn some more of the lingo, before you really understand what they are talking about. Start looking through the threads, and you will start to figure out what the differences are, which ones you may like, what others like, etc. Then just choose a few, and flash away.

3) Completely depends on how long the device keeps developer interest, and whether HTC actively supports the device and (as mentioned) releases kernel source for the device for the given Android build.

For example, if HTC decides not to release KLP for the device, and doesn't release kernel source, it makes it really difficult for devs to make custom ROMs with all the hardware components working properly (cameras, Bluetooth, etc.). But its not impossible, and if some devs are still inspired, they may try to make things work as much as they have time for.
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Phone: HTC One X (AT&T version) Rooted, Bootloader Unlocked, S-Off

Tablet: Samsung Galaxy Note 8 (WiFi only) Rooted