Firstly, I would recommend you get an FC-7 pedal. I find running a G70 without one very difficult, as there are not enough dedicated buttons on the front panel, and my hands already are busy enough!
To a certain extent, the RT*M advice is good. Most of your operational requests ARE spelled out fairly clearly in there. For instance, saving an HB (B3) section preset, (p.34), etc.. The index is your best friend! But be careful! The G70 has gone through two MAJOR upgrades, and several of it's more important new functions are not in the main manual (not unless they have reprinted it since OS1, which I haven't heard about). There are PDF's at Roland (and roland-arranger.com) for both OS2 and OS3 additions. If you don't have these, yes, it may be confusing to find out some info!
A 'stop' or Break function can be set to go there, or can be assigned to the ASSIGN SW (reasonably convenient for the LH). This is on p.18 of the OS2 addendum.
To use Intro 3 and then go to VAR 1, you press VAR 1 button, then Intro button, then VAR 3 button (button 1 should flash). You then press Start or Sync Start, and bob's your uncle! (p.26-27 of the main manual)
The Drawbar setting can either be stored to the registration (UPG) you are working in (p.110), or if you need a library of presets to chose while you are in one UPG, you can store the HB presets and overwrite the ROM ones (p.34 main manual). But be warned, these presets are stored in the UPS, so must be re-created for each UPS (User Program Set, a sort of master list of 192 registrations).
Copying Parts for assembling new styles in in the Style Composer section of the manual (p.185 main manual)
Most of your questions seem to involve using the G70 in a 'live panel' mode, which, although do-able, runs counter to the way that Roland have slanted the OS. They REALLY work their best when you knuckle down and create a specific UPG for each and every song, or AT LEAST each and every way you prefer to use the keyboard. And things like the OTS have their own specific behavior... they are basically over-riding UPGs, and as such, you can't just hold the LH. Sometimes, with such a specific need for a certain tune (you don't want to hold the LH Parts for ALL songs, do you?), you can write a pair of UPGs, and place them next to each other in the Set List, and switch seamlessly.
There's probably a couple or more ways to achieve almost anything you want on a G70, but being darn near Roland's flagship (the E80 is based entirely on the G70 OS, with some newer additions), it is a VERY deep and intricate machine. A lot of your requests seem simply answered, others are more complex, but you sound like you have come from another manufacturer's OS, and somehow expect the Roland to do exactly the same thing, exactly the same way. Unfortunately, it doesn't work that way, my friend...
You move from ANY manufacturer to another, and you are going to have a shift in thinking and workflow. And, unfortunately, reading the manual carefully, staying up-to-date with all the addenda, and spending quite some time with the arranger and manual in hand, investigating all it's intricacies is part of the process. Whether Roland, Korg, Ketron or Yamaha (amongst others!) is what you came FROM, any other is going to involve the same amount (roughly!) of study. Several of your questions were answered with a simple look at the manuals' indexes, and there it is!
About the only way you can move to a TOTL arranger from another and NOT have to go through this is to move from one model to another of the same manufacturer, and even that (say you go from a PSR2000 to a T2) is going to involve more than a month of (not
) reading the manual methodically..!
As to the styles... well, AFAIK, no arranger has the perfect style for EVERY song, and coming from another, you always have your ears attuned to one style. But either you CAN find something that WORKS (even if it is not the same), or there are a wealth of older G1000 and earlier styles, and conversions from other manufacturers out there (maybe even the style you USED to play it on!). You just (like the rest of us) have to go out and find them...
Don't be in such a rush to change, or you'll find yourself returning to where you came from, and obviously, you DID hear something about the G70 you liked! If you need change, if you need a different sound, that ALWAYS comes at a price, no matter WHAT arranger you change to!
Don't give up... things WILL get better!