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3. How long an exposure is too long for guiding though. 2 seconds?
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Do you mean for your guide camera? It depends on the accuracy of your mounts polar alignment and periodic error and the focal length you are imaging through. If you have good PA and very low periodic error imaging through your 8" you could easily go for a few sec.For a HEQ5 though I'd using an 8" newt you may find you are limited to only a sec or two. It will be trial and error I guess.
An xy guidier is a device that attaches to the back of your guidescope an allows you to move your guide camera up and down and left and right to find a star in the scopes field of view without having to move the scope. This works because your small camera chip is only sampling a very small section of the scopes possible field of view.
You could alway make yourself a set of guide rings. If you can get hold of a second hand set of standard rings that are a size or two larger then the scope, drill and tap three holes in each ring and then thread in three bolts to hold your scope. That is all the cheaper guidering are.
Can't comment on the quality of the 2" reducer, but it won't make much of a difference compared to a 1.25" reducer, you are still using the same size chip which will cover the same size field of view regardless of the reducer size.