Upgrading to SSD on 2010 MBP
re 1: hard to say - if you can spare the Money you should go for the 850 bc. of a (probably) better Garbage Collection (see #3), else go for the 840.

re 2: The best tutorial for replacing the optical drive with a harddrive (or SSD) can be found at iFixit.com, f.e. this one. You should search for the Tutorial for your model, the link i posted is probably not for your model.

re 3: There is a hot discussion going on if Trim should be enabled or not and if that is a bad thing. Fact is, Apple has Trim Support enabled only on its own OEM SSDs and disabled it for third party drive. There are Tools to enable TRIM Support, but it could be a not necessary bc. of a better Garbage Collection within the SSD Firmware (see this article and this Article). For general Infos i recommend this Article from Samsung 

BTW, to create a homemade Fusion Drive you need to use the 'Core Storage Volume' Command. You can look it up with 

man diskutil


look at the 'coreStorage' section. There are some tutorials for creating a homemade Fusion Drive, but it's a very painful process.