Not true, you could still load a full custom firmware with a9lh but nobody has made one yet aside from an early linux port that doesn't even have sd card write support much less wifi support. It is also no more or less patchable than a9lh (with the exception of ntrboothax) in that it will be removed during a system update unless you use a cfw with boot0/boot1 protection. The only real difference is that it is a cleaner and easier environment for cfw developers to work with.
1. Yes, unless the battery dies in the middle of it or something (hint: plug in your 3ds). Though it only takes a minute or so to do and most of that is safety checks. Even then if you have another 3ds, dsi, or even original ds you can use ntrboot to unbrick it.
2. Boot9strap/sighax are a cleaner and easier environment to work with, some of the advanced features of Luma 8.x and newer would require ugly and likely unstable hacks to make them work in a9lh.
3. No idea, possibly.
4. Yes, if you boot GW in sysnand mode on newer system versions (regardless of what boot hack you use) the hack will be removed as GW doesn't have boot0/boot1 protection. GW also doesn't support anything higher than system menu 11.3 11.2. I recommend you create and use an 11.3 11.2 emunand with gateway.