1. It depends on what you mean when you are referring to "secondary fermentation." See secondary fermentation is a bit of a misnomer, and causes confusion to many, especially new brewers.... As it's commonly used- racking a beer off the yeast and into another tank. To take it off the yeastcake and to let the beer clear. As most folks use it in this way, like I said it's a misnomer... because it's not about fermentation at this point....it's about a vessel called a secondary.
This is typical the Brite tank (as an alternative term) a place to let yeast settle out and bulk condition the beer. In fact many of us have adopted the term Brite tank instead of secondary fermenter to avoid the confusion.
This was typically done decades ago when yeast was less healthy than today and the fear was the yeast would autolyse and cause off flavors in beer... Although largely outdated idea wise, it was still passed on urban legend like from brewer to brewer but a decade ago many brewers (including a lot of us on here) began to question the wisdom of that, at least on a homebrew scale, and experiment, and found out that it wasn't necessary.... and many of us felt our beers actually improved with extended contact with the yeast.
This was met with a lot of resistance initially in online communities.... but some of the author who contributed to this actually came out and said that they just repeated the old outdated wisdom without getting it any thought... and more and more of them admitted they too opted for extednded primary conditioning and only secondaried when for example adding fruit.... You can see some of the early arguments and discussions in
this thread....
A true secondary "fermentation" is when another source of fermentables is added (either in primary for many folks nowadays or traditionally in a secondary VESSEL... this is usually something like Fruit... the sugars then ferment with the yeast, making it the SECOND fermentation of the beer... then the yeast is allowed to settle out.. or the beer is either racked to a seco0ndary if the frmentables were added in primary... or a tertiary if they were added in secondary.
Either process can apply to ales and to lagers... lagers are especially traditionally racked to a secondary vessel to clear and conditioning in the cold...This is the lagering phase.
2) Depends on the beers... Many commercial beers are fined and filtered and then force carbed.... they may or may not have been moved to a secondary vessel or brite tank.... Then they are forced carbed. Many other beers are bottlen conditioned.
3) Yes, although lesser known, ALES too can benefit from a diacetyl rest... About 8 years ago I posted some articles I had discovered about diacetyl rest and extended yeast contact in ales... the info was
posted in this thread. I think towards the back.
4) If I recall correctly the temps are discussed in the afore mentioned thread. I think
in this post. Unfortunately the links look to have expired in there... but the names of the sources I cited are there, and some dilligent searching online might allow you to locate them.....
The part of the Drayman's piece actually suggest you LOWER the temp slightly when doing a diacetyl rest with ales.
Best of luck