That's not how I read it. The "m" indicates it's mental, so mental defenses apply, but "m" does not inherently grant a RR. Otherwise in the case of Fm spells, you would get two RRs. So, I conclude Pm means you get a single RR, with the outcome of the RR defined by the P, which means you don't stop the spell but only detect it.
Ah, but how would you play out the "mental defenses apply" without a RR then, since most "mental defense" spells merely add a RR bonus?
I think Presence as U is reasonable. You are only getting a count (though some give more info), and since the spell costs no PP you cast it a lot. If you had to roll a RR for every person in the radius every time, that would be unwieldy.
P only allows RR for the target to detect whether he's subjected to such a spell, though, at GM's discretion, so, really, that wouldn't be unwieldy to leave it as P, as most targets wouldn't actually pay attention to whether they're being scryed on or not, or it's not important enough to bother. OTOH, in case it matters, well, a RR is allowed by the rules.
My main problem with defining
Presence as U, though, is that it breaks the definition for U: spells that "only affect the caster, a willing target, or a target incapable of resistance". Uh?