Yeah, I mean you're right as far as your PnL.
I guess it depends on what you mean by "lose." I want to buy @ 100 but the ask is 101. My market order fills at 101. In my mind, I have already "lost" one tick on the entry.
The inside bid/offer doesn't move and suddenly I want out @ 101 but with a market order, I'm filled @ 100. That's my second tick loss although the PnL will just read a loss of one tick.
Assuming a loss of two ticks per round trip is a good approach if you're backtesting an ES algo, for example. If you have a more sophisticated slippage model then you might not have to be so conservative.
you're mixing up "last traded price" with bid/ask. market might be down to 100/101 and price might never even reach 100 bid, so placing market buy order and being filled at 101 could get you in at the lowest tick, therefore your "I have already lost one tick on the entry" is wrong.
you cross spread once per RT on market orders. you are always free to place limit orders and get in line to get filled (or not).