You can thank the NES for the 30-row support (and a few other enhancements). Keep in mind that when all 30-rows are enabled, the name tables are larger. Pixel scrolling is also available in T80 mode, but not the multiple pages. Unlock the F18A and set bit >40 in VR49 (as shown above, thanks PeteE), and you will have 30-rows available. The Text modes also have the "position-based attributes" available.
Note that in the next F18A firmware (yes, there is going to be at least one more, if not two, updates), Sprite Linking and the Real Sprite Y position features are going away. I only mention those here because they are part of VR49.
Not AFAIK, which is not really definitive I know. The reason for the change it technical and I needed to remove something to get FPGA resource utilization below 99%. It is common for software that uses multiple spites (like games) keep a copy of the sprite table in CPU RAM and just overwrite the whole table in VRAM when a sprite needs to be updated.
F18A settings survive a warm reset, as well as cartridge inserts (when doing so does not lock up the console). It is just the way it all worked out. I suppose this could be handy for making a utility that can set certain F18A features for various reasons. For example, the virtual scan lines, sprites per line, and other such settings are all software controllable regardless of the jumper settings (the jumpers are just the power-on defaults). So a utility could let the user turn those (and other) features on or off until the console is power-cycled. The palette registers are another example. You could set a completely new color palette if you want, which will survive until you power-cycle.
I'm glad others chimed in to help you, since I have literally forgotten about half of these things! Uh, I just make the tools, I don't actually *use* them...