Perhaps this diagram will shed some light. You should be able to click on the image to get more resolution.
If I understand your situation, your two KW variable throttles control the trains on two loops which I call Loop 1 (U-A) and Loop 2 (U-B). As discussed previously, you cannot use the KW's 14V fixed Accessory voltage for insulated-rail triggering since the 14V is between C and D...as opposed to between U and C, or between U and D.
As you correctly suggest, you can use a separate train transformer that puts out a fixed 14V AC Accessory voltage. By "separate" this means it has its own power cord that plugs into a wall outlet.
You then connect the new 14V common (often the "black" post) to the U post of the KW. In effect, this makes the black post equivalent to the KW's U post. Now you have fixed 14V between the new Accessory transformer's "hot" (often the "red" post) and the KW's U post.
So to answer your random questions:
Yes. The single Accessory transformer can power 2-wire and/or 3-wire accessories that are on either loop. Of course this assumes the Accessory transformer has enough power (Watts) to drive all your accessories.
Yes. It doesn't matter if the insulated-rail trigger section is on the inside or outside outer rail. I don't show full loops but this is shown on the right loop where there's one trigger section on the "top" side and another trigger section on the "bottom" side. This also means you can have multiple trigger sections around a loop. I show what I call "straps" or "jumpers" between the outer-rails of the regular track sections. By regular I mean NOT the insulated-rail trigger sections. Some track systems have these electrical connections built-in to regular sections of track; other track systems you must make these connections yourself.
And, if not obvious, you can have multiple accessories triggered by one insulated-rail section. This is shown on the left loop where you have one 2-wire accessory and one 3-wire accessory triggered by a single insulated-rail section.
