Depends on your team strategy and gameplay as a whole. For example, if your hard carry (e.g. pl/drow) is having a difficult time farming, it is advised you go for a BoT to gank/backup/ cause a distraction in a different lane. If your lanes are doing well treads is preferable. You can generally tell how good your hard carry is doing by minute 10. If <30 cs and no kills he is not doing well at all.
Or on a question of strategy, sometimes you can have a strat which aims to gain advantage by towering early and putting pressure on all lanes (e.g. Pugna, sf, Furion, rhasta, sven (we used this)) all are decent/great pushers individually. However here sf is also the main carry so he goes BoT to farm up as well.
Only if you're not doing well and getting ganked a lot. Massing either (usually 2/3) is fine ONLY if you're not doing well. Heck, you can even consider a vanguard if you want to save space.
Bottle is core. SF should really be rune-whoring. magic wand is a bonus.
Desolator is best mid-game, late-game after damage items (butterfly/skadi at the very least. better if you have something extras like buriza) satanic wins. Judge if you can finish by mid or late. The damage item is even more so important because if a hard carry like pl has radiance and butterfly, satanic alone would not save you.
Depending on how you play and the line-up. Lothars is better for "catching", some dps and counterganking. If you're doing well and planning to stick to a lane, go lothars. Blink is a great all-around. It is arguably better for escaping (this would require you to know how to juke for 3 seconds if you have received damage) and is instant placement. If my team has an AoE disable (tide,enigma, es, tree, etc) I usually go for blink.
Always, unless you're stomping then you can do whatever seems reasonable.
SF is greatest at following levels (assuming your opponent is equal level or lower): 7, then 11-16. 7 is when you max raze and 11-16 when you have level 2 RoS. Level 1 RoS is kind of pathetic and not really worth it.
If by some chance you are outlaned and falling in level (due to ganks etc.). Farm, farm, farm! Close the gaps in level. Farm the lane, farm the nc and if you can get some ganks farm their heroes. If the opponent team is teamfight oriented and pushes towers as 5 ignore them and go farm and steal THEIR towers because unless you can land a good ulti and several razes in a teamfight you're no different from a support.
Stats if you need more mana/ if you are dying a bit too much. Otherwise PotDL.
Hope this helps