1. Yes low pressure and zonal marking don't make you lose much condition, but keep in mind that you have to play based on your opponent team's quality, if he has a much stronger team he will probably destroy you with those tactics, sometimes you have to spend those packs or use your reserves.
2. No bonuses don't tire your player more, they supposedly increase the efficiency of your team in that area.
3. I don't fully understand your question but yes younger players train faster and their ability to grow slows down with age. Not every player has the same ability, some grow faster(see fast trainers posts on the forum) and some don't.
4. No you can't manually put players on negotiations even thou we have been demanding that for some time. Your friend or anyone who sees your player as below 99% can bid for him directly but that is very rare and not the same as regular negotiations.