My take on the slats is that they were to distribute the weight of whatever was put over the coffin. Wood would eventually collapse and IMO this coffin maker needed the outside elements to hold until the remains were dust, for his/her own peace of mind. With such great care given to the whole thing, I believe whoever did this didn't want to think of a collapse. It's possible this wasn't the only location considered, and large rocks or soil would've been even heavier. Especially if the location chosen had been somewhere wetter and therefore quicker to rot the wood.
Red is a powerful color and would be chosen intentionally. There are myriad reasons for red, from ethnic to spiritual to symbolic, but again someone put in the effort.
As for being so close to the road, the coffin presumably had to be carried by humans. So a 'good' spot would be far enough away from the road, but not too far to carry. If the intent was to visit in the future, it had to be accessible but not strangely far out. Someone coming upon the 'visit' might just see someone at the side of the road who stopped to check a tire, take a bio break, etc.
As for the visual, if she was found with the bag over her head from a self-inflicted gunshot, the person who found her likely didn't want to look and left the bag in place. The same scenario would hold with an assisted or mercy killing, or even a murder. If she was found after hours had passed, that bag might have been 'held' to her head with congealed blood. Either way, the finder or shooter was probably a) in shock even if not surprised, and b) wanted to "fix it" quickly.
LE probably knows more info based on graphic tells, i.e. an xray showing hair pins means this was a mess. I'd bet there were other things in the coffin that are being held from the public. But the goal is to first find out who she is.