imo you'd be better doing something like

X15: Newcastle - NON STOP (Mafeking R'bout (for trains), Regent Centre and Gosforth Only) - Morpeth - Direct via A1 - Alnwick - X15 Route - Berwick (Every 2 Hours)
X16: Newcastle - NON STOP (Mafeking R'bout (for trains), Regent Centre and Gosforth Only) - Morpeth - Direct via A1 - Alnwick - X18 Route - Berwick (Every 2 Hours, Newcastle to Alnwick, Extended Every 4 Hours (Winter) / 2 Hours (Summer) to Berwick/Belford)
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X15/X16: Super Express Newcastle - Morpeth - Alnwick (Every Hour)

X17: Newcastle - X15 Route to Shilbottle via Stannington - 472 Route to Alnwick omit. St James Estate Blakelaw Road loop - 473 Route to Wooler (Every 2 Hours)
X17A: Newcastle - X15 Route to Shilbottle via Stannington - 472 Shilbottle Loop - 472 Route to Lower Barrasdale omit. St James Estate Blakelaw Road loop (Every 2 Hours)
X18: Current X18, Curtailed at Alnwick (Every Hour)
X19: Current X14 (Every Hour)
X20: No Changes (Every Hour)
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X17/X17A/X18/X19: Limited Stop Express, Newcastle - Morpeth (Every 20 Minutes)
X18/X20: Coast (Every 30 Minutes)

471: Rothbury to St James Estate Blakelaw Road, do loop (Current Frequency)

X16: Withdrawn completely, daily extension covered by X17
472: Withdrawn completely, covered by X17A
473: Withdrawn completely, convered by X17

At the end of the day, people will prefer the links to Newcastle rather than Shilbottle and the X18 doubling back is completely stupid regardless. Alnwick to Morpeth deserves 2 buses an hour, including one which is super express and it's pretty much there anyway with the X16/472 doing most of it. 

Glen Valley appear to have used coaches on the 472/473 before aswell so no reason why a full size bus can't do the X17 routes.