I used to live in BRU until recently and will agree that connections between the two concourses do require a long walk. However, compared to LHR for example the process is pretty straightforward (no bus rides and the queues do not get out of control). There are also travelators most of the way (not fast but they do make things easier).

I also admit that the walking distance is especially bad for US-bound flights as these depart from the very end of the B Concourse. Unfortunately this is largely due to the outcome of discussions between the Belgian aviation authorities and their US counterparts, who inspect all overseas airports with direct US flights periodically and insist on certain security measures. In order to minimise inconvenience to those not travelling to the US, BRU has created a sealed-off area at the end of the B Concourse (flights to TLV also use this) with a secondary security check. DUS has done the same for its US flights. I do not enjoy this check but it is no worse than that in place at FRA, for example. DUB had an inspection last year and afterwards was forced to adopt checks that created lines that almost overwhelmed the departure concourse. The UK impose more stringent security checks on everyone from the outset so do not require such intense secondary security, though even they have toughened up recently, resulting in much longer queues than is usual.

In short, I sympathise with you but the security issues and the long walks are not unique to BRU.

Turning to lounge access, I am not familiar with what an AAdvantage elite card gets you, but if you did not have your current EXP card with you at BRU then I'm not surprised that you were not given an invite by the check-in staff. SN have no access to current data on who has what status with AA. I have elite status with a Star Alliance carrier and I am never allowed in a lounge without showing my card, whatever my boarding pass may say, even by the airline who issues that card!

Once the BA lounge re-opens these problems should go away, as you will not need a lounge invite to access the SN lounge. The BA lounge is in any case nicer than the SN lounge IMHO.

Finally, on a pedantic note, the carrier you used to fly to BRU was SN Brussels Airlines, or "Brussels Airlines", or "SNBA". I have never, until today, seen it called "Brussels Air".