Then she will remove her thong!

Does he just...go for it? Regrettably she does not know the sign for 'go ahead'.

He says something that might be like "please" or "with your permission," based on the context that she's heard, and then pauses looking up at her.

She's being pretty clear here, but, like, it's a little weird to touch someone who hasn't said a word to you. Maybe she kinks on it?

In that case, she will initiate the touching! She'll put her fingers on his shoulder delicately and then drag them upward towards his neck, then lift them up and towards herself, ending at around her abdomen. That's probably not a grammatically licit sign but it's probably understandable? She remains slightly smiling. She has perfect control over all her facial and bodily expressions and they might have different ones here, but she's trying to project the vibe of pleasedness and interestedness.

Okay, good enough for him! Maybe she's feeling tushot in a way that doesn't make sex aversive or something.

He rises to follow her motion, pulling his body closer to hers and brushing against her legs. He plants a kiss on her collar bone and then pulls back, to see if this was the correct course of action.

She smiles and looks lovingly at him. They can do foreplay! She puts her hand on his shoulder again and drags it across his shoulderblades and down his arm.

Okay, this is actually weirdly stressful, because now he can't tell if that was a caress or if she wants him to lean sideways for some reason.

Probably the responsible thing to do in this situation is back off and establish better communications. Þereminians are all about being sensible and backing off and trying to establish better communications.

But he's horny, she's gorgeous, and this isn't doing great things for the sensibility of his decision making process.

He trails kisses up her collar bone and to her neck, applying just a bit of suction. Then he tries placing his hands on her side — with another pause to check that this is permissible — and running them over her skin, feeling its softness.

Dorcas has already started putting her tendrils in him, though even without this, she can detect the hesitation. How do you sign 'you can do whatever you want with me and I'll be fine with it'? Regrettably, she does not know. When he hesitates putting his hands on her sides, she puts her own hands on top of his and presses them gently towards her skin. Hopefully that signals that it's fine.

Yup!

With that confirmation, he will happily continue kissing and caressing her for long moments, before eventually trailing a line of kisses down across her breasts and stomach, to her inner thigh, and then to the hood of her clit.

He gently pulls back her labia, and leans in to wrap his lips around her clit, gently suckling it to full stiffness and trying to judge her level of arousal, and which sensations she likes best, here.

It's obvious, to someone watching his reactions with carnation, that he thinks this is great. It's an enjoyable, well-defined task that he can do without thinking about spreadsheets. And it's just hot and nice to be kneeling between her legs like this.

Her tendrils don't allow her to read his mind; not in the sense of a magical thought detection spell. But with her tendrils in him, she can watch every synapse firing and every neurotransmitter being secreted and every muscle flexing and contracting, with precision sufficient to resolve individual proteins. And she can indeed tell that he's enjoying it a lot! She is too.

She can control the stiffness of her clitoris or her lubrication, but she doesn't want to, here. She will let it happen without trying to use her powers on herself. She puts her hand on his shoulder again (wow she's really relying on that too much) and she gently pushes or pulls depending on whether or not she wants him to keep doing what he's doing.

... if he tentatively moves her hand to his head can he get her to grip his hair and use it to steer him appropriately?

In the meantime he catalogues her reactions and engages in the kind of almost-unconscious relaxing optimization of someone who's had a good deal of practice at this. He'll focus on her clit with light, unhurried stimulation until she's sufficiently lubricated to probably find fingers enjoyable, and then slip a pair inside of her to curl up against the spot where the nerves from the clit pass close to the walls of the vagina, rubbing in synchronization with his suckling.

She moves the hand on his shoulder around slightly and she notices him react when she gets closer to his scalp. And when she notices him liking that, she moves it closer and closer until it's at the back of his head. She doesn't grip his hair but she does detect that she can use a little more force in steering.

She sighs deeply when he puts his fingers in and curls, relaxing palpably. He's good at this! Top decile of people who have ever eaten her out. She can tell that he's very attuned to her, and that he can tell when she likes what he's doing. 

Then he will happily continue focusing on her pleasure until she reaches that floating and trembly state that presages overstimulation, or until the alarm on his phone goes off in about an hour. In either case, he pulls back with one last set of kisses to her inner thighs, bows from his kneeling position, and says something fast and possibly ritual.

It won't take a whole hour! But at the end of it, she smiles down at him again and pats him on the head.

Hm...she'll try signing too. She does the 'B' handshape pointing at him, then the index finger curling towards herself, and then a fist pumping forward and back in front of her open mouth. And then she tilts her head. Does he look confused or does it look like he gets what she's trying to say?

She wants to do a role reversal (as indicated by her swapping the honorifics on the pronouns) and return the favor? Sure!

He hops to his feet and adjusts his sporran and the flap of his kilt to expose his underwear, which proves to have a cunning flap in it for just such a purpose as this.

Thus prepared, he runs a hand through her hair and grips it, not so tightly that she can't move her head a bit, but tightly enough to indicate that he's setting the pace here. Then he brings her face close to his member (quite erect, from his earlier enjoyment; clean, uncircumcised, and of an entirely average size) and waits to see what she does.

(It's common knowledge that people are biased toward submission, but that obviously results in imbalances in the number of available doms. Standard advice for people having to play at being a dom to match their partner's needs is to think about what you would want them to do in the reverse situation and then do that. That and a bit of acting goes a long way. Most people aren't obligate subs, so sex is still enjoyable, it's just another thing to think about.)