Sometimes. Not all the time. I read the first post and generally I just want to answer before reading the other replies. I usually do read the other replies after I post my thoughts.
Yep. If it's a zillion page one, I read the last 5-10 pages. Conversation shifts and often the topic in the title and the first post isn't what's being discussed anymore or a clarification changes what you want to say about it.
I chose other because I have never paid attention to it. How much I read prior to posting depends on what the topic is. For posts asking an opinion on something, I usually read the original post then give my reply prior to reading any other responses then go back and read to see what others have said. Sometimes that means editing my reply to account for new information provided by the OP at some point in the comments. Threads where the OP is looking for advice (example being what to look for in a good vacuum or where to buy diamonds online), I usually read most or all of the comments and add my voice in support or a new option if I have anything to add. Long threads, I tend to read the last page or two (or more) then jump in if I have anything to say.
If it’s a question / advice thread, then I read all the replies (at least 90% of the time). If it’s a SMTB I usually see the first post and then reply, but I scroll past all the other posts in case there are more pics etc.