Then there's recoil: Well established (in books, WH40K rulebook background, and FFG rulebooks) an Astartes bolter would likely tear off the shoulder of a normal human. Bummer.
Well, I would say it is not "well established" just because a handful outsourced books propagate this. Let's face it, 40k is not one singular setting, and I could counter the impression of recoil by pointing towards GW's own books (Codex fluff as well as the Inquisitor rulebook), as well as pointing out the relation between low initial velocity + heavy gun weight.
Law and religious superstition are much better arguments, however. Given how Space Marines seem to be held by the general population, I could well imagine the average Guardsman (if there is such a thing) not feeling well about touching one such relic, especially if it has lain dormant in the tomb of some great hero for possibly hundreds or thousands of years. On the other hand, in the midst of a crisis, I think some types of characters could be pushed far enough as to "violate" the sanctity of the place in this way, with the pure intent of using these armaments to fight the filth that dared invade one of the Emperor's worlds. Whether or not his superiors, the Inquisition and/or the Chapter the Marine was a member of agree with the individual trooper's assessment is another story, of course.
The safety is a good point as well, although here I would not be sure if any and all patterns of the gun would actually have it. This is something where the story could come first, just like in various novels.
Another possibility would be to have just some of these weapons from the tomb work, of course. The Guardsmen could find out that they don't get the bolter to fire, for example, but the power sword might still work nicely!
Objection! Astartes weapons are only slightly heavier than their normal counterparts. Human Bolter weights 7kgs. Legion/Astartes Bolter weights 10kgs. The biggest difference is between the human Power Fist (13kgs) and the Legion/Astartes Power Fist (25kgs).
It seems the weight of these weapons seems to jump up and down depending on the game one is looking at (it was a whopping 18 kg in Deathwatch!), just like the penalties for using one as a normal human, or what damage they do. In GW's Inquisitor, all bolters were the same and had a weight level of 40 (which meant that a character below Strength 40, well beyond of what average human characters would start with there, would suffer a BS penalty).
Ultimately, there's no reason not to tinker with these weapons - provided the end result is something all players in the group feel comfortable with. Just like GW Codex X or Black Library novel Y, FFG is not the sole authority on Wh40k when it comes to detail, and you as the player have just as much creative control over your interpretation of the setting as whoever writes the many contradictory books one can read.
In the end, regardless how you turn it, an Astartes bolter is less bulky and fires less large ammunition at a much lesser rate of fire than a heavy bolter used by normal people - even within the RAW of FFG's games! Make of that what you will.