Buckthorn Replacement Questions

I just bought a house with a nice wooded back however as you can see in the first pic, it was horribly overgrown in most sections with buckthorn. I have been clearing the area and as you can see in the second pic how beautiful the area could be. I'm a little worried about other invasives coming in now with more light and more space and am looking for help. I'm a first year gardener and I've been reading about permaculture a lot. I've read plants like raspberries & blueberries (which I love) are not only nitrogen fixing plants but will also keep buckthorn and other invasives at bay. Is this accurate & do I plant them wherever the buckthorn was (if it is I will be close to a commercial scale with as much of it was there)? Will planting clover over the area help keep invasives away? I also have about 20 comfrey (bocking 14) root cuttings starting in buckets. I am located in Minnesota.
Thanks in advance!

I'm currently working through 10 acres of buckthorn with varying success. I think I can help a bit.
raspberries and blueberries are not nitrogen fixers, raspberries may grow well enough to contend with the buckthorn, but blueberries likely won't.
If you killed the roots of the buckthorn or removed them you can pretty much plant anything, though if you simply cut it down you will have regrowth and a thick mess within a year or so.
Cover crop like clovers do seem to contend with the seedlings, I would recommend a mix of clover, some sort of annual grass(wheat, rye, etc), and something like radish or turnip. A deer food plot mix would work well so long as the grass included is annual.  
The comfrey will help, but might not grow fast enough right away to suppress buckthorn seedlings and certainly won't compete with trees.
Fast growing trees like boxelder, elderberry, and black locust can help fill spacer temporarily and could be chopped and dropped later. Walnut seems to compete pretty well also. Weeds like Burdock have out competed buckthorn in places for me too.

Your second picture looks pretty clear, with very few seedlings or stumps around, you probably can plant whatever you want back there, I would guess you have other areas not as cleanly cleared and those are going to be tougher to reclaim.

Keep at it, we cleared most of our backyard and it has remained clear with many fun natives just showing up out of no where. After the buckthorn has been removed from the ecosystem it seems like alot of healing happens quickly. We've found hazelnut, highbush cranberry, dogwood, birch, maple, currant, raspberries, and even things like bloodroot, solomon's seal, and jack in the pulpit simply appear in recently cleared areas.

Good luck!

Here's a thread on some of my observations.
https://permies.com/t/42073/forest-garden/Buckthorn-Whitetail-deer-tough-situation

The berries are totally not N-fixers, but raspberries [and even moreso for blackberries] can make a heck of a potent but messy and challenging to harvest ground cover.

If you want n-fixing fruit, what you want are Eleagnus species such as Goumi, Autumn Olive [possibly illegal due to invasiveness claims in Michigan?] and Seaberry [ironically possessing the other common name of Sea Buckthorn, despite being of no relation whatsoever to common buckthorn.]

As to the question of keeping 'invasives' away, you have to ask yourself whether you want more production or more work. More light getting into the lower lays gives more yields, but part of that yield [until you get every scrap of soil filled with living roots and covered by 2+ layers of shade] is going to be 'weeds.' One mentality that helps is viewing those 'weeds' as a resource, organic matter for the soil or biomass for heating/cooking.

In the case of Buckthorn specifically, you'd probably want to rogue through for regrowth in the following spring and then again late summer. This should eventually wane down to once per year and perhaps someday down the road it may stop coming back. [I say may because Common Buckthorn is fairly shade tolerant, anything with enough light penetration for most berry crops (perhaps excluding Currants/Gooseberries) is probably going to provide enough light for it to grow albeit perhaps at a reduced rate.]