From what I can see, your best bet is to purchase an off peak return from Machynlleth to Shrewsbury for £21.45 with the railcard, and then if travelling to Wem, paying £5.60 for a single from Shrewsbury to Wem (if travelling just to Shrewsbury, this ticket is obviously not needed and it saves £5.60). Then, for travel on Sunday, there are two options, depending on what time you want to return at, as it is slightly cheaper to stick to purely trains that stop at Wem on your journey back.
If yes, then you can buy a Wem to St Helens Central anytime day return for £22.80, and then you will just need a £5.60 single from Wem to Shrewsbury on your journey back to complete the journey, as you can use the first ticket from Shrewsbury to Machynlleth. However, this restricts you to the 15:42 and 17:42 departures from St Helens Central, which would get you to Machynlleth at 19:41 and 22:40 respectively.
If you wish to take the 16:42 or 18:42 service back from St Helens Central (which I imagine would be the most ideal) which arrive back at Machynlleth at 20:42 and 22:40 respectively, you will need to instead purchase a Shrewsbury to Crewe off peak day return for £13.80 (and use it starting short at Wem for the outbound) and then an off peak day return from Crewe to St Helens Central for £16 - this costs £1.40 more than the journey above, but allows travel at any time.
Unless you are certain you want to take the 15:42 or 17:42, I'd recommend paying the £1.40 extra, and then the total is £51.25 if going to Shrewsbury on sat or £56.85 if going to Wem.
