The Liverpool Street - Ipswich hourly stoppers are non-reservable. Then either an Ipswich-Cambridge or Ipswich-Peterborough as far as Stowmarket.
So you had in mind, instead of taking the suggested 2110 service as per the original itinerary, the customer could reasonably argue the following are appropriate connecting services...
Stansted 1900
Liverpool St 1947
Liverpool St 2002
Ipswich 2128
Ipswich 2219
Stowmarket 2235
Well, maybe, but I am not sure I would recommend making that argument.
So what then about from Stowmarket onwards?
Your booked train only.
When you book an Advance ticket, you can get almost any valid itinerary you want, but it is recommended to obtain your intended itinerary at the time of booking rather than argue a huge deviation is valid after purchase.
Which service would be best from London Liverpool Street in terms of getting a seat?
I don't think you'd have much choice other than the 2002; I do not know your chances of getting a seat. Staff might argue it's not valid as your itinerary was supposed to be on the 2110 from Stansted to Stowmarket.
The train tickets in GB are convoluted and not easy to understand.
If you want flexibility, get an Anytime or Off Peak ticket
Advance tickets are rather like airline tickets; you get the trains that are specified in your itinerary (unless delays occur).
So it's no more complicated than an airline ticket.
The rail industry does offer some flexibility in terms of your connecting service from Stansted to Stowmarket, but I do not see how that can be seen as adding complexity. I guess they could be stricter and say you had to get the specific connecting service on your itinerary, to match the strict conditions of airline tickets, if air passengers felt that would make things simpler. I'd be careful what you wish for