I'm thinking your problem is that Michigan Bulb sent plants that were too young to be viable. Lavender is not a commonly bare rooted plant...Not that I know of anyway. I have had extremely bad luck with bare rooted perennials from mail order, packages, boxes etc. Not only have most died or never grown at all, many were not what they were supposed to be. I no longer will buy them. I'll pay more for pots and more established plants so perhaps I'm biased. OK, I know I am biased.

Dig a hole where your thinking about putting the draining pipe,fill up with water and see how fast it drains,repeat a couple times,you'll have your answer then.

Lavender isn't easy to grow in the colder climates. Try planting it in a protected area (for example, near a south wall). Drainage is very important but if the soil doesn't drain well, amending the soil in a pocket where something is planted won't help drainage; it will simply create a "sink" where water pools. If your soil doesn't drain well, you'll have to find a way to get water away from the planting site. If your soil drains ok, I'd leave that part alone. As mentioned, lavender doesn't like rich soil or fertilizer and doesn't like much water so don't fertilize it and rarely water it (if at all in your area -- I water mine here a few times during the summer but we get less than an inch of rain over July and August -- if you get a good soaking every now and then from rain, you might not need to water it at all). I have some lavender in a planter that faces west and gets hot afternoon sun. The planter is full of garden soil and some rotted compost (no sand but it drains well) and the lavender is thriving. But the Northwest is a great climate for lavender -- I never could grow it well when I lived in Indiana. If you don't have a good spot in your garden, try growing it in a pot. It'll need to be protected in winter -- put it in a garage that stays cool but doesn't get below freezing, for example. Container growing might be your best alternative if you can protect it in winter. Good luck!

One thing about OP's planters-they are quite deep. As such, it is entirely possible, even likely, that roots will never get out of that "vault". I'm talking soil oxygen. Down deep, there usually isn't any and where there's no O2, roots don't grow. So any chance of "escape" may be only a dream. I've got a whole business district/main commercial street of this city in which such vaults were (accidentally) built for the trees. To translate that to English, we re-did our main drag some 15 years ago. I was on the committee as was our city forester. He and I both called for curbing, to keep the constant flow of salty water (winter de-icers) out of them all winter. so far, so good. What we did not spec was 4-foot frost footings-necessary in this climate for foundations and such. We got them anyway. Those trees are trapped in these structures-no roots can ever make their way out. For me-the guy that has to try to grow annuals and perennials in and around these trees, it has become hellish, the entire "soil" in these things being nearly 100% tree roots. Bad for the trees, bad for the ornamental plantings. Anyway, Just wanted to point out that at the depth these things are built, no root growth beyond the perimeter is likely.