My 2 pence worth.
I did a lot Google searches and research for fitting an early Lotus +2 (Capri) windscreen.
Many results were returned.
Firstly as many know, there is a lot of information on this subject on this Forum; but the search engine on the forum is not the best; but a google search on a main browser always returns much improved results particularly for the forum.
A simple summary suggested the main methods used were to fit the seal to the body then the glass and the other; as you have noted; to fit the seal to the glass and then fit the unit to the body.
As my seal was a single length (not joined) and that I would be fitting it single handed, my choice was to fit the seal to the body, then fit the glass then the St.St. trim.
Carefully check the rubber seal for burrs and the trim groove to ensure that it?s all open, so that the ?hooked angle? will fit in it. I read somewhere that someone was at the point of fitting the St.St. trim and found some part of the grove was sealed. They had to start again. 
The first seal length fitted ran along the bottom of the body aperture, with the correct angle cut at each end, with a very sharp broad blade tool. This was cut approx 15 mm longer than required, as it is known that the rubber will shrink with age.
Then I cut a length that ran from the bottom corner around the sides and top and back to the bottom. Again with accurate cutting of the mating angles and approx 20 mm longer than required.
The rubbers were pushed firmly into the body aperture particularly around the corners and carefully checked then the angles were glued with a Loctite rubber adhesive. This was then left for 2 days.
Then using a silicon spray (or liquid soap or baby powder) liberally applied to the rubber I started to fit the glass.
Firstly along the bottom ensuring it was equal both sides, using a slight angled trim tool I pushed the glass into the rubber.
Next I went approx 2 thirds up one side, then fully up to the top on the other side. The last third up the side and finally along the top of the seal. The glass went into the seal with little difficulty, just a firmer hand on the corners.
Finally the St.St. Trim was fitted.
Before fitting the bottom piece I used a trim tool slightly larger than the thickness of the “angled trim” to run along the bottom of the grove for the trim, to ensure that the weight of the glass had not pushed too low into the seal.
Applied silicone spray liberally into the groove, then I pushed in the bottom trim.
Then the 2 bottom corner clips were pushed onto the bottom trim. Silicone spray into the grove and 1 side top trim pushed into the groove. A right angled trim tool was useful especially in the corners to fit into the groove to pull the rubber to allow the trim to be pushed into the rubber. Also a wood batten placed on the trim and rubber mallet to gently persuade the trim in.
The top centre clip was pushed onto the fitted trim passed the centre joint, the other side / top trim was fitted and the centre clip tapped into position.
It all worked for me. 