"sauntered through a top-heavy group with Belgium": what does a top-heavy group mean? is that top-heavy group referring to Belgium?
The first round of the World Cup involved each team playing all teams within their group. England were in a group with Belgium, Panama, Tunisia.
"Sauntered" literally means to "walk casually", so here it is being used metaphorically to imply that the England team easily and casually went through this round of the World Cup.
Something that is "top heavy" is literally heavier at the top than it is at the bottom, which can mean that something is (again, literally) unbalanced. When used figuratively it sometimes just means something is unevenly or unfairly weighted (not necessarily from the "top").
The implication here is that Belgium were the best team in that group and markedly better than everybody else, making the group "top heavy". So yes, Belgium are the "heavy" part of the group.
"conveniently dropping its final game against the Red Devils in a barely-contested 1-0 loss": who lost? England or Belgium?
England lost against Belgium (the "Red Devils"), but that didn't matter to England's progress in the World Cup as a whole because they had already scored enough wins/points in all the other matches in this round to go through to the next.
The loss to Belgium was "convenient" because if England had beaten them it would have meant the next round would have been more difficult. Some even suggest that a deliberate loss was a good tactical move in the competition.
Had England beaten Belgium, they would have faced Japan in the last 16 and then Brazil in the quarter-finals, whereas coming second meant they play Colombia and then whoever won the Sweden vs Switzerland match. (Sweden won.) England has lost to Brazil in previous World Cup matches.
"Because that put the unappreciated but savvy England manager Gareth Southgate and his team in the soft half of the incredibly lopsided knockout stage bracket.": this sentence doesn't look like a complete sentence. What does it mean "in the soft half of the incredibly lopsided knockout stage bracket"?
You're right, it isn't a complete sentence; it's a technical mistake that breaks up a longer sentence for effect. It really follows on from the thought of the previous sentence. It would have been better phrased:
...conveniently dropping its final game against the Red Devils in a barely-contested 1-0 loss, putting the unappreciated but savvy England manager Gareth Southgate and his team in the soft half of the incredibly lopsided knockout stage bracket.
This really reiterates my answer to your second question about why their loss against Belgium was "convenient". It placed England in a knockout stage that was much easier to win in. Interestingly they use the word "lopsided" here, which is similar to "top-heavy", but better describes something with just two adjacent sides where one is heavier (or lower) than the other. The "soft half" describes the side with the weaker teams in it.