Thanks for the photo, it does clearly show the form of the spacing block between the two rails. This was the part of the chair that I was uncertain about.
...As keith said normally you would fit the stock rail first, though some track builders fit the check rail before the stock rail (Gordon S for one). I am told that the check rail gauge is more important than the stock rail.
In the past when using standard chairs I leave 2 sleepers without chairs where the check rail goes and only fit 2 chairs on the check rail. I then trim back slightly the 3 chairs on the stockrails and 2 on the check rails where they touch each other, replace the missing chairs by cutting them in half and glue in place.
That's exactly what I do with chaired track, though I normally use ply not plastic timbers, and I've always found the method quite satisfactory. You are right that the check rail to crossing gauge is the important one. I leave fixing many of the stock rail chairs until after the check rail has been gauged and stuck down, then set the gap to the stock rail before fixing the stock rail and adding the half chairs.
Once the chairs have been glued in place they are stuck fast, providing they are in gauge when the solvent dries they will not move. Plenty of turnouts have been built over the years to prove the point. Some do use the odd copperclad sleeper on slips etc, where the chairs have to be cut about. With the advent of the Exactoscale special slip chairs this should not be needed (once I have finished the catch point I will be building a Timbertracks single slip using this method)...
Maybe I wasn't clear enough. I wasn't questioning the strength of the chair/timber joints or, indeed, the strength of ABS chairs assembled from multiple parts. I've built enough turnouts this way to know how strong they are and am often amused by the rivet and pcb fraternities who go on about glued chairs not being strong enough. Sometimes I wonder just what they expect to be able to do to their track.
Instead, my point was questioning the dimensional stability of ABS when subjected to butanone. The check rail chairs may be made to give the correct gap, but unless held in place by gauges when the butanone is applied, I wouldn't want to rely in the ABS alone to maintain the correct spacing. In a similar vein, some folk will tell us that because the normal chairs are made to give the 1:20 inwards lean of the rails, you will automatically end up with the prototypical lean. In fact, if the rails are held using simple roller gauges, they will end up vertical because the butanone is quite capable of softening the chair so the rail adopts the alignment of the gauge, not the chair. In fact, I've used this effect when building early inside-keyed track to ensure that the rails do not lean outwards due to the shape of the chairs.
Another subsidiary question was really what you thought the benefit of using these chairs was in your case. I can see the attraction if you are building track with 4 hole chairs but, after all, you are throwing two-thirds of them away.
ps. Keith, 'sloshing' was perhaps a slight exaggeration but from several years experience with ply timbers, which I much prefer to plastic, a fair amount of butanone is required to get the chairs well keyed into the wood, probably more than is needed with ABS 'timbers'. I suspect that a failure to appreciate this may be why some people have reported getting poor ABS/wood joints. Certainly, my sloshing is not enough to remove any detail. In either case, though, the chair is sitting in a butanone-rich atmosphere for a reasonable period of time which I reckon must lead to some overall softening and so the potential for distortion. As said above, there are even cases where this can be turned into an advantage.