Depends on how good you are with basement stars ... The reason LBLJ is faster is because basement has faster stars with certain strats and because there is less doors you have to open with stars. If you can do the basement stars fast enough that is when lblj starts being worth it. Some of the fast basement star strats are pretty hard, though (pillarless, red hot log rolling without wingcap, lavaboost ...).

Exactly. The only important thing is you collected all of the blue coins in one session at least once and then got a star after getting the last blue coin. Also SBLJ is not allowed as category extensions go by 16 Star rules unless specified otherwise.

I guess what you mean is something like enter level, get plant coins and then cannonless, reenter level, get the switch coins and then wild blue. This would not be allowed, as the rules state "All blue coins in a given stage must be collected in one segment and you must get a star after all blues have been collected.". One segment means you have to enter the level, get all blues, then get a star. Reentering the level to get another star will not require you to collect the blue coins again, though.