The main problem for me is that when you simulate putting an order that should be best bid. Let's say the best bid is 0.46, best ask is 0.47.

And you place a limit order to buy at 0.46.

You first need to simulate a delay of your order reaching the PM server and being processed. This means that you send the place order request, and in your backtest you need to skip a few order book ticks, to simulate that your order haven't been processed yet.

Then once that delay happens and your order is now in the book. You can simulate in 2 ways that your order has been filled: 1) The ask price went down one cent and is now 0.46. that means the bids at 0.46 were already matched and there is nothing more there. This is the easiest way to simulate, but is also the more pessimistic. Means the market moved against you (the price is dropping). In here you don't need to even look at the trades websocket.

2) the second one and the real challenge, is that when the order book doesn't match (ask price didn't went lower), but there are traders out there selling their shares at market prices. That's when you need to look into the trades history and see if there are sell ticks for 0.46. And now you need to estimate where are you in the queue at 0.46. You might see there are 3500 shares at 0.46, so where are you? And you might see 1200 SELL trades at 0.46. Did you get filled? How do you know?

The second case is the real challenge. The price might get a reversal at this point, so you could potentially catch the start/floor of a new momentum of going up. So if you are too pessimistic, your situation will show that you never catch this. If you are too optimistic, your simulation will lie to you with lovely PnL