Please ask a priest for a Catholic perspective on this. A forum won't get you the answers you want.

It is my understanding that such activity should come to its natural conclusion; otherwise it's sinful.

3. This is what an old priest told my sister on the subject: "Do what you want in the bedroom, swing from a trapeze if you please, but it must end in intercourse."  Otherwise it is a mortal sin.

This corresponds to everything I have heard as well.

Foreplay is ordered toward intercourse, so it is unnatural to employ it without that end attempted.  As you noticed, there are not any general health risks with intercourse during pregnancy. So just proceed as usual.

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I disagree.
What and who defines excessive?
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Well, you'd be wrong.  It's the same principle that applies to eating.  If you're excessively indulging in the pleasure of eating, then you commit the sin of gluttony.  There's similarly the possibility of engaging in a kind of sɛҳuąƖ gluttony, where the pleasure is out or proportion with the spiritual/intellectual ends of the marital act.  This is commonly taught by the Church Fathers and all subsequent theologians.

It's very vague to talk about the possibility of venial sin when "excessive pleasure" is sought after/experienced in sex.

Only thing that's vague is the discernment of when someone has crossed the line into sin.  As for the possibility of committing venial sin against either purity or in terms of gluttony, there's no question that this can happen.  If I sit down and stuff my face with two pounds of cake ... even to the detriment of my health, that is clearly the sin of gluttony, since the pleasure has clearly gotten blown out of proportion with the primary end of eating.  And St. Thomas, among others, teach gluttony can only be a venial sin.  Yes, we experience a certain amount of pleasure in eating food (and with sex), but the pleasure must have its ORDER.  And if it becomes disordered, where it dominates over the other ends, the Church Fathers and the majority of theologians agree that there can in fact be venial sin.  So in some cases, the question of venial sin might be vague, but in others it's very obvious (e.g. when I eat a couple pounds of cake to the point of making myself feel sick to my stomach).  Same principle applies to sex.  Sex can become a mortal sin because the end of procreation is much higher than that of feeding the body.  When you completely rule out the primary end, then it's mortal sin, but when the secondary ends to some degree become sought after more than the primary end, there's a degree of either imperfection or else venial sin, depending on how far it goes.  This is called DISORDER, and to the degree that it's deliberate, it can be venial sin.  To the degree that it's not deliberate, it's imperfection.  To say otherwise would be an outright denial of sin by disorder, and order/ends/means/disorder are THE KEY considerations in all Catholic moral theology.  Period.

If you're saying that "excessive pleasure" cannot be a mortal sin, then there's no such thing as the sin of gluttony.

"These excesses", as you call them, are precisely what are venial sins when deliberate or, otherwise, imperfections.