I am surprised you want to build a new railway. Especially when there is a crossover at Crow Nest junction so a reversal is quite possible.
Did a fair bit of thinking about potential timetabling on this kind of route, and adding that 5mins by the reversal and changing ends would make the benifits much more marginal, running non-stop to Crow's Next Jcn from Manchester is about 20 to 25mins from what I'd researched, with then roughly 5 to 7mins via Westhauton without traffic into Bolton.
A single track curve or twin track curve would be preferable because once you start pushing past the 40min mark in the off peak times a rail replacement bus would be able to keep up with the likely timings of a half hourly diverted service, making the cost of setting up the service a tad too much just for peak services.
An alternative that I'd come up with was total possession of one tunnel at a time and new crossovers being put into place to the south of Kearsley, and bi-directional running installed between there and Bolton, with a 5mph TSR going past the worksite, timetabling might be able to be pushed up to having 3tph per direction, but that would be pushing it, so 2tphpd would probably be more likely.
Looking through a quick draft I did a while back the timetable would look something like a core of:
xx:05 Bolton - Manchester Oxford Road / Manchester Piccadilly
Calling: Salford Crescent, Deansgate, (Manchester Oxford Road)
(Clears single line section by xx:15)
xx:35 Preston - Manchester Airport
Calling: Buckshaw, Chorley, Adlington, Blackrod, Horwich, Lostock, Bolton, Salford Crescent, Deansgate, Manchester Oxford Road, Manchester Piccadilly, Manchester Airport
(Clears single line section by xx:45)
xx:01 (Off Piccadilly) Manchester Airport - Preston (Taking the path of the Current Liverpool L St Service)
Calling: Manchester Piccadilly, Manchester Oxford Road, Deansgate, Salford Crescent, Bolton, Lostock, Horwich Parkway, Blackrod, Adlington, Buckshaw, Preston
xx:35 Manchester Oxford Road - Bolton
All operated by 6 car units, in the peaks there will be extra services to Victoria flighted behind the others, calling at Moses Gate, Farnworth and Kearsley southbound in the AM peak and northbound in the PM peak ONLY, off peak, services operated by rail replacement buses to intermediate stations, all tickets accepted on local bus services, with two additional peak services, they would call at alternate stations.
Wigan had a 2tph shuttle in place, Clitheroe terminates at Bolton, Todmorden Curve used to accomadate Blackburn.