This question implies that front-running prevents execution. It does not. Tesla's NBBO often has a wide spread. Firms that pay for order flow can easily receive orders that are between the NBBO spread, including orders that cross.
Example: NBB is 312.32, NBO is 312.82. Order flow comes in to a firm paying a broker to route to them. First with a limit sell of 312.40, followed by a market buy 930 microseconds later. The order flow recipient front-runs these orders, selling to the market buy for 312.81 and immediately buying from the limit 312.40 sell order, pocketing 0.41 risk free in the process. Both public parties got execution. The market buyer got a price better than the national offer at 312.82 so they could be happy and certainly can't be mad. The limit sell got their order filled right away too at their limit so they aren't complaining either. This is probably not illegal but it is still front running because the firm that paid for the order flow used a millisecond or so to re-order (in time) these crossed orders and place their own in-between for a risk-free profit. It is obvious that this order flow is non-public information. If it was required to be routed directly to NASDAQ, the national best offer would have dropped to 312.40, and the market order would have filled at 312.40. The order flow firm, not participating would get nothing and the market buyer would have gotten a better price.
Order flow firms don't front every order, only those that they are certain or predict will be profitable for them. For the order flow that doesn't cross at a risk-less profit, the order-low firm forwards it to another venue, or perhaps executes some for their own account depending on other expectations they have.
Notice that the buyer still got to buy the shares, thus "accumulates their long position" even though it cost them more than it would if no front-running were happening.
It is quite easy to observe trades that could fit this pattern when the volume is slow during the mid-day. There is often multiple trades at diferent prices taking place between the spread as the bid and offer remain unchanged. It's also true that some customers could be led to believe that they got "better" pricing than the spread because of intermediaries. But in this example, it is not because of the wonderful work of the middle man, it is in spite of it. I understand that you say there are no HFT firms that pay for order flow, so this can't happen at any HFT firm. I'll take your word for it.
But it is very peculiar that you try to tell us that a front-run customer could not get an order filled. Why would you say that?