I expect a proffessional to know theses things and to ask questions. Has anyone read the "Goods Act"...you are entitled to rely on the skill and eexpertise of agents of any product...not read minds but ask questions!
your Caveat Emptor approch would not be tolerated in any other industry. When you pick up your next new car and they tell you after you paid that the engine is extra and there is no way out will you be happy???
Again, a fairly selective example. It would have been better to say the purchased car only had 300litres of storage space as opposed to 350litres, and expecting the salesperson to have pointed that out to you... of course not. They don't know that this is a relevant point to you.
A TA would have known the baggage allowance differences. You would have only had to point that this was important to you. A TA is usually happy to book you on a more expensive fare. However, the TA isn't going to sit down and say "seat pitch on three options are xx_, transit times are 10 minutes more for xx_, entertainment options are xx_, meal options are xx_, transiting airports for an older person then xx_ airline is better." They would have said something for sure if it was a LCC, but Qatar is an excellent (overall) airline.
You tell the TA what you want and what is important to you. In this case "most direct flight and price". Then you have got the result you wanted because this is the cheapest and most direct.
As previously mentioned, if you're over by a couple of kgs, they will usually let it slide. Especially for someone who looks like they aren't able to juggle 7kgs in hand luggage. Also, look at pre booked excess luggage as well.
Anyway, good to see you have moved on from blaming Qatar to blaming the TA.