The Python-based installer is a long story. To long to mention (and I'm still looking at alternatives). As for your questions:
- LUKS - for the time being Sparky doesn't support it; It's a quite exotic option
to a common user, so it wasn't a priority.
- LVM - the installer doesn't support it; The current installers are simple scripts from other distros or projects and we needed something to fill the gap.
- separate boot partition - that's supported by the Python installer, although not specifically mentioned; You just create the partitions yourself and assign them mount points by right-clicking on them in the installer (/, /home, /boot, etc.). The sparkyinstaller-gui (the fallback installer, as I like to call it) doesn't have such options.
- GRUB installation - again the Python-based installer has that, but the fallback tries to do it on it's own.
- reserverd blocks - neither of the available installers supports it (see: exotic option
- >2TB - I have no idea if this works... None of us has some much disk space.
Both installers come from different projects (the Python-based comes from Mint and the fallback is just remastersys modified by Pavroo). Answering your questions made me wonder about the sad state that the Sparky installer is in. I've been looking at an alternative for some time. It's called Calamares and it attracted a lot of attention lately (maybe you've read about it), but it'll still take some time to sort this whole mess out. For the time being we try to fix problems as they come. There's only two of us, so it takes some time.