Island sink vent loop questions

Hello,
I'm planning the DWV plumbing for a kitchen island sink and have a few questions about the requisite vent loop. We're on the 2006 UPC here and air admittance valves are not allowed. I'm using the following image as a reference: <
http://www.rd.com/images/tfhimport/2003/20031001_Ask_Handyman_page001img001_size2.jpg
(1) The drain line will be 2". Is it necessary or useful to enlarge the vent from the usual 1.5" to 2"?
(2) I assume that there is no horizontal separation requirement for the two vertical legs of the loop. So from the fixture san-T I can come up to a street 45, a 90, and then a street 45. Is that correct? Also, is that a vent 90 or a long-turn 90?
(3) Is it important that the vent leg of the loop connect to the drain downstream of the drainage leg of the loop, rather than upstream?
(4) To get both the horizontal vent and the horizontal drain to fit in the joist cavity, I'd like to minimize the height difference between them. In usual pattern, the vertical vent leg comes down into a san-T for the horizontal vent take-off followed by a combo to tie into the horizontal drain. I'd like to roll this san-T/combo over to 45 degrees from the vertical, by inserting a 45 elbow before the san-T. Is there any problem with this? I assume that the prohibition on "flat venting" would prohibit me from rolling this over any further.
FWIW, there will be a regularly vented sink upstream of the island sink, and I plan to run the island vent over to the this area to connect with the regular sink vent. Thanks in advance.
Cheers, Wayne
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I think I found the answer to this one: vents have to take off downstream of the trap. I'm not sure why that is, but it is easy enough to follow.
Thanks, Wayne
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OK, I think I've answered all my own questions except one. So:

<
http://www.rd.com/images/tfhimport/2003/20031001_Ask_Handyman_page001img001_size2.jpg
Not necessary, and not useful given concern (4) below.

UPC 909 says I can use a one piece return fitting; otherwise I have to use the 45-90-45 pattern. The 90 is a drainage 90, but need not be long-turn. Nothing wrong with using street fittings.

Yes, the vent always attaches downstream of the trap.

Actually, for the UPC, the vertical vent leg has to come down to a combo for the horizontal vent take-off.

Still not sure of this, whether I can roll the combo-combo over 45 degrees. Any comments? Or any other ideas on minimizing the height difference between the horizontal vent (foot vent) and the horizontal drain?

No other fixtures can be attached upstream of the loop vent, so I have to split my drain line downstream of the loop vent to accomodate the other sink.
Cheers, Wayne
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