What happens if service is not clear?

Rule 8.4 covers what happens if the defendant fails to file an acknowledgement of service:

8.4

(1) This rule applies where –

(a) the defendant has failed to file an acknowledgment of service; and

(b) the time period for doing so has expired.

(2) The defendant may attend the hearing of the claim but may not take part in the hearing unless the court gives permission.

It may be possible that the defendant has a good reason for failing to file the acknowledgement of service, in which case the court may allow the defendant to participate in the hearing like a normal case. Otherwise, the hearing would proceed and the case would be decided as normal.

The reason you can't request default judgement per Part 12 when making a Part 8 claim is because of the nature of Part 8 claims:

[where the claimant] seeks the court’s decision on a question which is unlikely to involve a substantial dispute of fact

The intended purpose of Part 8 is to seek answers to questions. For example, seeking guidance from the court on the proposed actions of trustees or what the correct interpretation of a deed of trust is. There's no real injustice to be remedied for the successful party in such cases—as opposed to most Part 7 claims, where the successful party usually wants a financial award or some other material remedy—so default judgement would be inappropriate to grant.

Would it be likely that one would win a successful judgement against a defendant at an incorrect address?

This is a mere technical defect to cure. The court would simply direct that efforts be made to ascertain the correct whereabouts of the defendant and a new notice of claim would be served upon them. The case would be adjourned until reasonable efforts to serve the claim were made. It is not a pre-requisite that the service be successful, otherwise people up and down the country could simply claim not to have been served and that's not in the interests of justice.

would the defendant likely request that the judgement be set aside and the case started anew, once they had "discovered" my pending action?

Assuming that, for whatever reason, service was unsuccessful, and you managed to obtain default judgement (under Part 7 since Part 8 no longer applies here because Section 214 claims no longer need to be made under Part 8 per Part 56), the defendant could apply to have the judgement set aside but whether that would be granted is highly fact-specific.

It would be perverse for a defendant to deliberately claim to be ignorant of a claim against them and seek to "go back in time" for their own advantage without compelling reasons why. The claimant deserves justice equally as much as the defendant does.