Although I cannot rule out that there maybe alliances that use your so-called "mixed alliance sandbagging" as a mask to do the sandbagging that is causing the issue, I do not agree that widespread "mixed alliance sandbagging" is an issue itself, rather that's an unpreventable outcome of the inability for allisnces to actively recruit new members within the game, which is a different issue all together. (yes, alliance can recruit externally, i.e. on the forums or other platforms, but it's obviously not efficient)
The intention of doing that is not to win WW, at least for the alliances that I'm in. All alliances have to recruit new members in order to grow and be sustainable, older members would sooner or later quit the game, which is a typical scenario in gaming (you wouldn't play a game forever, right?). Most alliances would love to recruit high Age, high Level and experienced players, but the reality is: it isn't that easy nor are there so many of those active players that are searching to join an alliance. So, alliances would have to recruit lower Age, lower Level and less experienced players, helping them progress and make their way up. In return these players would eventually help the alliance grow. The alternative is to start a feeder-alliance for less experienced players to progress before joining the parent-alliance, but not all alliances can afford to do that.
Extending the Top Alliances ranking list(it used to list the top 500 alliances a year ago) does ease the issue of difficulties in recruiting but it is a very inactive way of recruiting, and it doesn't solve the sandbagging issue nor does it target your so-called "mixed alliance sandbagging". The only positive effect of extending the ranking list is to make it more convenient to search for an alliance.
I agree that the game is off balanced, making players, those that know how it works(anyway all players that play this game long enough would eventually know), focus on offensive upgrades rather than other aspects. Regardless of whether sandbagging is an issue, or which kinds of sandbagging is an issue, something fundamental has to be altered to promote a fairer and more balanced gameplay, not only in WW but as a whole.