This seems like a fun adventure. I'll say upfront that I've never tried what I'm about to suggest.
When I read your question title the first thing that came to mind is a Puerto Rican sandwich that is very popular in Chicago, the jibarito. It's a sandwich in which the bread has been replaced by flattened and fried plantains. I think doing something similar could yield nice results for a pie crust. You just press the peeled plantains between two cutting boards to get your desired thickness.
As far as not frying, you should also be aware that fried plantains are damn good. I've never eaten a plantain that wasn't fried. That said, if you are committed to not frying I have suggestions as well.
First, the boiling and mashing is right out. I don't think the processing you'll be doing will result in a crust that holds up at all.
Roasting, or better yet, pan frying in butter I think is the way to go. I suggest just greasing a large frying pan with butter and pan-frying the plantain over medium-low heat until done.
If pan-frying is not your thing either, I'd suggest just brushing them with melted butter and tossing in a 400 F (200 C) oven for 10-15 minutes (complete SWAG - never oven roasted plantain). With either the roasting or pan-frying method you may wish to sprinkle each buttered side with a little sugar to add a little sweetness and a nice carmelization.
There are some previous questions with good answers like this one, or this that go into much more detail than I'm about to.
You said your bread didn't rise with the first rise, if you don't have a first rise you can't go onto the next step. If your yeast was old, or the temperature was cold in your kitchen or you used cold water in the dough, or the yeast came in contact with salt then its action could have been slowed down drastically and it would take much longer to get a rise. I've had a first rise take up to 4 hours in the past because of these factors. The times given in recipes are just guidelines, you have to be result-driven. Active dry yeast also takes longer than quick yeast to activate. When using active dry instead of quick yeast you would get better results by mixing it with the water before mixing in. I'd recommend using quick yeast as it's milled into smaller grains and you can mix it directly into the flour.
Of course your yeast may have simply been dead, or killed by salt or heat. Get some good fresh yeast and store it in the fridge to keep it fresh long-term.
Also, using all whole wheat flour isn't going to produce anything like a baguette. Whole wheat flour is very heavy compared to white flour and will take longer to rise. A half and half mix would work better. I'd recommend sticking to the recipe first, then gradually modifying it over time until you get your desired result.
Best Answer
Make sure you use a pan with high enough sides so that fat doesn't overflow. If it's a thin baking sheet with only short edges, especially one that tends to curve/warp in the oven, you could easily dump fat all over the bottom of the oven. But as long as it's a sturdy pan, the fat isn't going to get any higher than the beef itself started out (except for splattering), so if the beef itself all fits in with a bit of space at the top, you'll be fine. If it seems like you're cutting it close, split it into two batches.
Make sure also not to cover too tightly; you want steam to be able to escape. You want it to be covered well enough to keep fat from splattering everywhere, and to keep it from drying out fast and burning on the top or bottom, but you also don't want all the water to get stuck in there until it boils over.
Letting it cook in the fat is really just a personal preference. If you want the final result to be super lean and maybe a bit dry, don't do it; if you want to keep it moist and juicy and don't mind some fat, cooking it in the fat is good for it.
For food safety issues...
All that matters is what temperature the beef reaches. The FDA recommends that it reach 160F. You don't really need a thermometer to check in this case, though: if it's all obviously brown it's fine.
So, yes, that general idea is probably safe, but I wouldn't try to pin the time down now. Just put it in the oven and check periodically until it's done. Exactly how long it takes will depend on how thick the beef is in the pan (i.e. how big the pan is).