I mean only issue I'd have with doing an inspection before an OTP is, as a seller if someone does that and then submits and offer, I might be less likely to negotiate with them as they've already spent a few thousand. I'd take the chance they're emotionally and financially invested and would be less likely to walk away. The other side is if I get two identical offers I'd go with the one that did the inspection.
Saying the interest rate needs to be acceptable is fine and actually protects the buyer more than the seller I reckon. No-ones (or very few people are) going to come for you because you declined a prime interest rate - you showed your commitment in applying for the loan and all that. But again most aren't not going to buy a house because your interest rate is 0.25% higher than you wanted. Like if all banks come back with a rate within 0.5% of each other, that's where you're at.
Now if you get cold feet and want to use the rate as a get-out-jail card, I reckon you still could. I question how many folk will bother trying to force you to go ahead; maybe agents will want some sort of compensation I guess.