I never have anyone foam roll the lower back, and don't do it myself. However, i'm a big fan of foam rolling.
There's a couple of reasons I don't.
-What "feels" tight and what benefits from soft tissue work can be completely different. Loads of people complain of hamstring tightness yet, even if they use a PVC pipe to roll them, they won't feel much of anything going on (typically, anyway). Conversely, not many people "feel" tight in the quads and hip flexors yet these are almost always the worse (therefore, best!) areas to foam roll (btw - ITBand is quad tightness).
The lower back is typically similar to this. Also, I train my massage therapist. She's worked on my lower back before but it's never too bad. She says although loads of people get relief from lower back work (it "feels" nice), she doesn't often feel that "gristly" knotted tissue like she normally gets in quads, hip flexors, traps, pecs, etc.
In other words the tightness can be "Neural". Although to confuse things even more, you can say everything is neural...
It seems like the muscles that are chronically shortened (pics, upper traps, hip flexors, etc) are the one's that benefit more from soft tissue work where as muscles that are chronically lengthened (hamstrings, lower back) don't seem to get much from it.
However, i'm far from a soft tissue expert.... This is just my observation.
-You only have a couple of degrees of rotation between each vertebrae in the lower back before you hit End ROM. By wriggling around on a foam roller, it would be very easy to move into end range. This is when you start stressing discs, ligaments and cartilage. Similar negative effects to stretching the lower back. This could probably be avoided if you were careful I guess but, what's the point messing around with it when the problem is the hips anyway?
On your issue with Pulls from blocks - these can be awkward when you first do them. I think your back probably just needs a break. May simply be a case of too much too soon.
I've heard of people struggling more from mid shin than from the floor - maybe this is a sticking point.
Also, and not to step on the toes of your trainer. If you are doing pulls from blocks, rack pulls, or whatever, to improve your deadlift then you should aim to make the bar path as similar as possible. IN this regard, a common mistake is to set up exactly like you would a DL from the floor. You can end up with the bar too far in front, hips to low, attempting to squat the thing to get it moving, which makes you fall forward and hang off the lower back to make the lift.
If you actually were DLing from the floor, by the time you had pulled the bar a few inches (to your new starting point off the matts/blocks/whatever), some things would of changed - your hips would be higher for a start. So, it could be a form thing. As a general rule of thumb, you want to walk right up to the bar and touch with the shins before you move down and grab it. This for most people sets you up closer to the bar than when pulling from the floor, which would have your hips a little higher.
I'm not explaining this well, what I mean is you want your rack pull/block pull starting position to look your like your deadlift from the floor when it gets to that point in the ROM.
Also, that point in the lift is where people want to round over. Normally the hips shoot up as the bar breaks the floor then they round the spine about mid shin (of course, it differs, so this is just my observations). Really focus on getting the lats tight before you pull. During a DL from the floor your lats are either going nuts at this point, or they've died and your rounding the back.