-Does doing a small spray of saline spray right after a line flush out or help absorption? Does doing a small spray before help?
Yes, and yes. It does not help absorption, though. It will "flush" the remaining drug down the tube (your throat) and the remainder that wasn't absorbed in the nose will effectively turn into an oral dose after reaching the stomach.
Snorting creates micro lesions, inflammation, which both dry out your nose. Saline helps clean and helps the body repair this damage. It does not prevent permanent damage, but helps a lot to minimize it.
Saline before the session minimizes chance of infection. Think of it like this, rubbing sandpaper on your arm would hurt more if your skin was bone dry versus moisturized with lotion, yeah?
-Should you immediately feel the burn of the drug or does it take time sometimes?
Every drug is different, but in general if it burns it should be immediate or within a minute or so. If you feel a delayed burning pain this is more indicative of a dried out nose or damage you've already done to your nose, and a sign you should use saline or stop snorting and switch to a different ROA.
-is it bad to press my nostril together after a line?
It probably doesn't do any good, especially with something like meth, this will only cause you to bleed more. It is very unlikely to increase absorption a significant enough amount to warrant doing it.
-how often in a sesh would you flush out your nose?(say your doing it for 5 hours)
Optimally you should flush your nose after every line, but that's probably not gonna happen knowing most people.
Bare minimum you should at least flush when you are done with your session, and pre-flushing before starting. This is a reasonable amount of harm reduction and easy to do.
If your nose starts bleeding, feels inflammed or swollen, stop snorting. This is potentially causing permanent damage. If you want a reason not to ignore this advice just google "cocaine nose damage".
I don't believe ketamine is known to be particularly damaging to the nose, but a cutting agent could be.