hi! glad you joined....i think i can answer some of your questions.
answer #1. 4x4 is electronic, so here is how it works. when in 2wd the rear wheels will only get the power, the fronts will not! when you are in 4x4 mode, the machine will stay in 2wd until the rear wheels slip. it takes 1/5 of one wheel revolution before the front wheels engage, fully locked, so you get a true 4x4. much different and much better the limited slip front diff like in trucks, suvs, and other brands of atvs. as soon as the rear wheels regain traction, the fronts release again. so, on the trail, and in 4x4 mode, your machine could be going in and out of 4x4 hundreds of time, and you never know it. all you know is 4x4 is there when you need it, and not there when you dont. which basically answers the second part. you can put her in high, 4x4 and give her hell all day my friend. by the unit not being locked in 4x4 all the time, to makes steering much easier and helps save tire wear.
answer#2: yeah, 1500 rpm is an acceptable rpm for the clutches to engage. if it were say, over 2,500 or 3,000rpm you would have a problem. they have a CVT clutch system. you have your primary clutch, which is connected to you crank, and your secondary clutch which is connected to your gearbox(tranny) what happens when you give it gas is the primary starts to spin faster. there are weights inside the clutch that get thrown out by centrifugal force, and the primary basically grabs the belt and starts to spin it. the spining belt turns the secondary which turns the gears in the tranny and down to the wheels. it does take some RPM to get that primary to engage, because when stopped at idle, the belt tension must be loose enough the primary can spin without grabbing the belt. it does take some getting use to but thats because they are nothing like a car, truck, or motorcycle
answer#3 if you plan on doing a lot of your own manitence, ect. then yes, a service manual is well worth it. if you are just going to do basic service items, then i wouldnt waste your money on one. if you are mechanically inclined and have a good tool selection and a very good basical knowledge of all things mechanical, the a service manual will help guide you step by step through the little qwirks of your machine and show you EXACTLY how it should come apart and go back together. an OEM service manual is going to run you about $50-$80 price range, or you can get an aftermarket one which are just as good IMO. i have included a link for you for an aftermarket service manual if you are interested!
Haynes Service Manual