In the final layout there are three routes from Reading West towards the main station. The existing up Westbury will have a straight run towards P8, joining the down Reading Festival Line (which is an extension of the P8 down main loop) just prior to the Caversham Rd bridge. There's also a route into P7. (As I understand it these are the only routes available to get to the east of the station in January.) I've not mentioned P1 and P2 which are unchanged, and P3 also becomes available again for XC reversing to/from Basingstoke.
After Easter there will also be the two new 'Feeder lines' through the new underpass, the 'Reading Feeder Main' runs directly into P11, with additional routes into P10 and P12 (but only to P12). It is bidirectional and available for down moves towards Reading West, e.g. for XC reversals from up main towards Basingstoke.
The other feeder line, the 'Reading Feeder Relief', is also bidirectional, and accessible in the up direction via a crossover at the start of the west curve, it also joins the line into P12, but using the west end crossovers can access P13, 14 and 15, all three of which provide a route to the 'Southern underpass'. I don't think there is a route from P12 to the underpass - hence your proposed through service towards Wokingham will have to use the Reading Feeder Relief. Hope that all makes sense without a drawing...
The Feeder line names appear to have changed slightly in recent weeks, originally they were the 'Down Reading Feeder Line (from P12 etc), and the Up Reading Feeder Line (to P11 etc). I think the new naming better explains that they are both bi-di.