Avanti staff can only get discounts for themselves and their immediate family (spouse/cohabiting partner and children/dependents), or for other people who are travelling together with them. So I'm afraid that even if a member of staff was willing to help you out, there isn't anything they could offer you.
Travelling with West Midlands Trains south of Crewe is likely to be the cheapest option, although it's obviously slower and less convenient. It's cheapest of all if you use Northern or TfW between Manchester and Crewe, but their first trains of the day wouldn't get you into London until after 9am. If you pay a little bit extra to travel with Avanti on that part, you can take the 05:05 from Piccadilly and change onto WMT at Crewe to reach Euston at 08:05.
It's also worth noting that even if you're not eligible for any age-based Railcards, you can still buy a Network Railcard for £30, or an annual season ticket that qualifies for a Gold Card for around £200. These would respectively provide discounts on your evening journey between London & Milton Keynes or London & Stafford (albeit the latter only on WMT, not on Avanti). You'd want split tickets at Milton Keynes or Stafford to get maximum use of those Railcards.
Another option worth considering is to buy a Manchester to Rugby Off-Peak Return, since this is effectively valid at any time (technically "not before 04:15"!). You could then combine this with Rugby-Milton Keynes and Milton Keynes-Euston tickets and use it on trains that stop at those stations (e.g. the 05:05 from Piccadilly, or 15:53 from Euston). If you buy a Network Railcard, this would cost £139.45 per return journey. Some trains in the evening peak have pick up/set down only restrictions at Milton Keynes or Rugby (although the "set down" calls at Rugby are advertised as normal calls at the station), so you'd want to buy a slightly different combination for those trains.
Finally, a slightly 'left field' option would be to consider flying from Manchester to Heathrow. If you buy the flights at least a month or so in advance they can be quite affordable, typically being around £90-100 return - even on 'commuter' timed flights. For the cheapest fares you'd want to 'nest' it so that each booking includes a Saturday night stay - so you might make one booking going out MAN-LHR on a Tuesday & returning LHR-MAN the following Thursday, and another booking going out LHR-MAN on a Thursday & returning MAN-LHR the following Tuesday, and so on.
Tickets for the Heathrow Express cost a fixed £10 each way if you buy them at least 45 days in advance, or alternatively you can buy a period return at any time for £39. The Elizabeth and Piccadilly lines are also slower alternatives that work out cheaper under most circumstances.