The problem with your assessment concerning the argument that mainstream feminists allow radical feminists to make the statements and advocacy they do because the mainstreamers actively benefit from it, has a lot of credibility.
Radical feminists are almost never lambasted and opposed by mainstream feminists and of those times when they are, almost never visibly or in a public way.
The closest you get to it is feminism being compared to "a tree with many branches". That analogy by its very nature makes it clear that while there is a desire for mainstream feminism to be distanced from the radicals, there is no desire for them to be separated from them since they do benefit from them.
Furthermore, the problem with feminism as a tree is its rotted trunk of a core concept of patriarchy. Now that concept can be excused in terms of what the early first wave feminists were looking at, since gender was a field of study in its superficial stages and the concept works in a superficial manner -- sort of.
However, the problem with the concept of patriarchy is that it completely ignores class and reduces all men to some all-powerful, never-victim, all-oppressing zeitgeist while depriving women of agency while reducing them to a singular zeitgeist of poor, pathetic, helpless damsels in distress locked in a big scary stone tower, guarded by a big scary dragon and waiting for a knight in shining armour to save them.
As a result, the concept of empowerment becomes utterly hypocritical because -- due to its grounding in the concept of patriarchy -- it becomes privilege without accountability.
These factors cloud every issue that feminism tries to involve itself in -- one of the most telling being domestic violence.
When feminists involve themselves in the issue of domestic violence, the results are utterly hypocritical. On one hand, they abhor and lambast female gender stereotypes directed at victims while actually reinforcing male gender stereotypes, either fabricating statistics or data outright, leaving death and bomb threats for experts in the field like Erin Pizzey and Richard Gelles and spouting utterly chauvinistic stereotypes towards male victims to justify it. They quote strength differences while ignoring the use of weapons and the fact that abuse can be more than physical. Furthermore, countless studies have found that roughly half of all domestic violence is mutual, roughly a quarter is exclusively women abusing men and roughly a quarter is exclusively women abusing men -- the very reason why Gelles et. al. have received multiple death threats.
When you example the core tenants of patriarchy, that's to be completely expected, because that is the conclusion: patriarchy, with its applied value judgments of "all men are abusers and all women are victims" reaches in the issue of domestic violence. This is just one example, but a highly telling one.
In fact, while feminism claims to be about equality, where has it taken a stand openly on issues where men are oppressed such as VAWA and primary aggressor laws, where police are trained to automatically arrest the man and not determine the perpetrator before making an arrest? The answer is nowhere, because it is only interested in helping women because under the notion of patriarchy, it assumes that only women will face inequality and men can never be victims of inequality.
Now, understand that I am far from saying that feminism is doomed to being an anachronistic and sexist ideology. However, it will require a radical paradigm shift to completely separate itself from the radical misandrists, comprehending the concept of hegemonic masculinities (John Tosh is an excellent source if anyone is interested) and understanding that men's oppression in society stems from their imposed expandability, and women's oppression stems from their imposed protectionism.