Helst_navngivet

Its enviromental condtions, but also whether the block its spawning in is valid or not. For most land animals this would be air and tall/short grass.
I am guessing the entity spawns are set in the "chunkdata" file, however i do not actually know.
hope this helps.

Im sorry to be the bearer of bad news, but im pretty sure its impossible to retrieve the boat and the loot inside EXCEPT if you are willing to get very wild and fancy, you might be able to fish out the loot in the game files VintagestoryData -> archives (dated to when its from, select the date BEFORE you lost it) -> should be there somewhere, but that requires a bunch of external programs (SQLite3, for example), and i can speak from experience that its a pain to download those. (I havent completed it yet)
if you manage to do all of those things, including deserializing the protobuf format/ painstakingly reading from the (decrypted) hex string (which requires DB browser for SQLite), then you can probably recover your loot by looking in the "EntityData" file/folder. Probably.
hope this helps.

i THINK i have found out where different colored seagrass spawns. I think.
turns out that the only mention of altcolor seagrass is in "aquatic-coral" which is kind of weird, but ill take it.
red, yellow and orange sea grass:
spawns in coral reef with RED & YELLOW with FIRE theme for DRY TROPICAL climates (thats how they typed it in the files)
min. temp: 25 | max. temp: 40 (tropical/near equator)
min. rainfall: 0 | max. rainfall: 1 (any rainfall)
min. water depth: 4 | max. water depth: 18 (can probably see the bottom for the most part)
if the files are correct, that means thats the only place you can get alt. colored seagrass. 
but they are in one spot, which is nice! (i guess)
hope this helps, and have fun!

as far as i know you can use any type of glass. However i have not tested it.
pretty sure it works for green glass, so it would be weird if it didnt for yellow.
hope this helps, and good luck on your greenhouse!