Building construction factors are level 3 optional rules that are not used in tournament play
I was under the impression that everything in Total Warfare was the standard rules. They start on page 166 and while there's a note that more advanced rules are coming in Tactical Operations I see nothing to say that the rules in Total Warfare are not the standard rules. In any case, this is a narrative not a tournament. The rules provide the setting detail that buildings in the setting are not universally built to support arbitrarily large weights on their roofs by providing rules for buildings that are not.
That's light cavalry. Heavy cavalry is for riding right over your lines in a welter of gore.
If that were what defined heavy cavalry the Highlander is not. With an LRM-20 and a Gauss Rifle that's every bit as much a sniper as the original Longbow. Using this definition thus invalidates your precedent for calling a 3/5/3 mech heavy cavalry. To me, the obvious conclusion is that whoever wrote that description of the Highlander had no clue what he was talking about. Sometimes FASA or Catalyst writers or freelancers make serious errors because they don't know what they're talking about and it doesn't get caught by editors who are also do not. If it was in the original publication of the Highlander laypeople simply didn't have the easy access to high quality military history that they do today.
The defining characteristic of cavalry is classically being on a horse, which obviously does not apply. The analogous definition when calling armored formations cavalry is being mobile over open terrain. The common line of battle mechs that have displaced infantry as the queen of battles are 4/6 slow mediums and lights like the Panther, Vindicator, Enforcer, and Centurion and for the Lyrans smaller heavies like the Thunderbolt. Calling a mech as slow or slower than these cavalry because it is designed for shock requires ignoring that shock infantry has been a thing for most of history until the widespread adoption of magazine rifles and smokeless powder, which is also when heavy cavalry stopped using that definition because running over enemy lines in a welter of gore stopped being achievable.
Any building insufficiently reinforced for it to walk on the rooftops is insufficiently reinforced to prevent it from playing Kool-Aid man and "Oh, yeah!"-ing right out the side for much the same result. And any decently old city in the Inner Sphere (which is most of them) is going to have a solid core of Star League era buildings that will be reinforced enough for roof hopping. The whole point of reconnaissance or planning is to discover/determine good ambush positions and attack routes that you can make use of.
If you could play Kool-Aid Man your jumpjets would not be enhancing your urban mobility because playing Kool-Aid Man does not use jumpjets. Either the risks are significant and being able to smash through buildings is not a substitute for being able to get over them or on top of them, or they're not significant. Jumpjets are exactly as significant to mobility through an urban environment as the downsides of simply barging through buildings.
Reconnaissance is not very helpful for determining which buildings it is safe to jump on unless it involves infiltrating the city planning office and getting copies of all the blueprints because the structural strength of a building is not likely to be apparent without detailed inspection by a civil engineer. And presuming that none of the buildings you expect to simply walk through have been renovated without permit to support something heavy high up or any you plan to jump on built below spec by the lowest bidder. The only way to be sure is to see a hostile mech of the same weight standing on top and that's not likely to happen since I think the heaviest jumper still in production is the Grasshopper.