The wording of the section on Mana Barriers is talking about magical force to break through the barrier, not physical force. The comparison it makes to any other physical barrier is to inform you to use the same system for tracking damage to the barrier.
On page 315 in the entry you mention there is a chart which describes the caster's options for where the barrier will be present, and lists the specific page governing the spell or effect which causes the barrier. Mana Barrier itself can be found on page 294 of the 5th Edition core rules, and describes a barrier which does not prevent the passage of non-living (physical objects) or living things unless they are 'magically active.' For comparison, the next listed spell is Physical Barrier.
Unbound/uncontrolled spirits active in the area are affected by the barrier and have to contend with it like anyone else. Whether or not a summoning could take place on the other side of the barrier would very much depend on the situation and the barrier.
If a mage (or other affected being) desires to pass or have an item which is affected pass through the mana barrier they have three choices:
The subtle way is trying to magically bypass it as described on page
316, where every additional hit allows them to slip other affected
items or beings through with them. This is described as pressing
through the barrier, much like Dune's 'the slow knife will penetrate
the shield.' This approach does not automatically notify the creator of the barrier
The forceful way is to target the barrier with damage-causing
magics. This is handled 'just as you would with any other barrier,'
by dealing with its Armor and Structure ratings. These ratings are
equal to the Force of the mana barrier. The barrier must be overcome
in one Combat Turn or else its Structure refreshes. This approach
automatically notifies the creator of the barrier.
The third option (page 317, sidebar) is to attack the source or
basis of the barrier. In other words you can deal with the anchor,
destroy the lodge, or geek the mage responsible for sustaining the
barrier. With the source out of the way, the barrier is no longer a
problem. This approach, if it brings down the barrier, will notify
the creator automatically.
These choices are the same regardless of where the barrier is on the Astral Plane, on the Physical Plane, or on both.
Mundane gear and allies are unaffected by Mana Barriers.
Affected beings or items cannot normally pass through the barrier, unless they Press past it or bring it down. In some cases a mundane object may force them through the barrier (page 316) in which case they must pass an Opposed Test or be disrupted.
The effects of disruption are listed in the Astral Intersections passage on page 316. Foci are deactivated, spells end, spirits are dispelled, and living beings are Stunned. Afterward, foci can be reactivated, spells can be cast again, spirits can be summoned again, and living beings will recover from Stun... providing the owner of the mana barrier gives them the time to do all of this.
No amount of time is listed as a penalty for passing through a mana barrier, just the condition resulting from that intersection. Using the details of that imposed condition indicates a deactivated focus can be reactivated by the person it is bonded to as a Simple Action - if they are conscious. Likewise, spells and spirits can become available again in the time it takes to call them forth. Living Beings will recover from Stun normally.
Basically: Not really, unless they're pretty sloppy and don't counter your Astral Tracking. Although spirits might not be as unstoppably dangerous as you think.
Any spirit influencing material matters will leave a material trace, just like any physical being - because they must materialize to affect the material world.
If a spirit wants to affect anything on the physical
plane, it has to materialize first (p. 314). It gets physical
attributes based on its type (Spirits, p. 303) and appears
as a solid, physical version of its astral form—it’s relatively
solid even if it doesn’t look solid, like a spirit whirlwind
or a water elemental.
The spirit is going to leave some kind of physical trace at the scene, and an astral impression wherever it travels. That will fade relatively quickly though, and the mage can wipe it clean - so you're right, it is quite a challenge to track a summoner who wants to stay hidden when you arrive on the scene way too late. Just as it would be hard to track down a Rigger who used a sniper drone, or to find a decker who stole your files, or to find a sneaky assassin, 8 hours after the event when they've covered their tracks.
You do, however have Astral Tracking to fall back on if the mage hasn't erased their tracks everywhere the spirit has been.
Furthermore it seems to me that the astral signature vanishes with the spirit.
I'm not seeing where that's the case in the rules. You've even got a "+1 for every hour since the link was active" mod in the tracking threshold table (p315), which indicates you can track stuff that happened in the past after the link is gone as well as things that are happening now. If a spirit was there 4 hours ago, for example, this would suggest you can trace its origin at a +4 threshold (and tracking master by spirit is a +0 modifier for bound spirits, or +2 if it wasn't bound).
On the theme of danger...
A nonawakened person is pretty much helpless against a spirit
This isn't necessarily the case.
The materialization thing means that even unawakened victims can defend themselves quite effectively. Once the spirit materializes to make its assault they can physically attack it just like any metahuman opponent. Spirits do have all manner of freaky powers, and can more easily sneak up on the unawakened, but they still go down if you pump them full of bullets. Even before they materialize you've got a chance of spotting them:
Physical beings may sense when an astral form passes
through their aura. Make a Perception + Intuition
(4) [Mental] Test—you receive a +2 dice pool bonus if
you’re Awakened. If the test is successful, the character
feels slightly breathless, and they experience a chill or
tingling sensation from the passing of the astral form.
Security personnel are trained to recognize this feeling
as a sign of an astral intruder.
Dangerous, yes, but not really any more so than other sneaky backstabbers who might try to kill you.
Best Answer
You can only have one unbound spirit summoned at a time
While the text on the book is vague about wether you can summon only one at once or control only one at once (core rulebook, page 300):
There is no mention about how many summoned spirits you can have at once, but there is about how many bound spirits you can have. Binding a spirit is much harder than summoning one, so why there would be a restriction on bound spirits but not on summoned ones?
This is enforced by the Additional Purchases and Resitrictions table (page 98), which says:
Previous Editions
This was the text on 4th edition (core rulebook, page 188) and it was crystal clear on how they are supposed to work:
The term unbound spirit isn't even used on the 5th edition core rulebook. But refers to spirits not bound to your character and there is no reason to believe this has changed between editions, as what is a bound spirit has the same definition (see the Binding skill).
Historically, this ruling has been the same on the 3rd, 2nd and 1st edition of the game. Shamans could summon nature spirits, while mages could summon elementals. Both also had limits, but they were mechanically different.
Nature spirits couldnt be bound to your character, their services had to be used while they were summoned and they would vanished on sunrise or sunset (similar to summoning spirits in 4th and 5th edition). While elementals had to be bound in order to be used, which took some hours and money (similarl to binding spirits in 4th and 5th edition), but would not disapear until all services were consumed.
The limitations were similar to what we got on 5th edition:
The significant differences were that nature spirits didn't require any preparation and could be used quickly, while elementals were stronger due to being on standby and materialize when needed, and had the option to follow more complex orders (Aid Sorcery, Aid Study, Remote Services, sustain spells, etc).
What actually happened on the 4th edition was that these rules were combined into a single subsystem that works for shamans and mages without major mechanical differences other than the type of spirits available and the flavor on how each tradition handles spirits.
Shadowrun Missions FAQ
This is also explicitly covered in the Missions FAQ (no official link yet, as their messageboards are offline, but here is an unofficial mirror):
As for the validity of the missions FAQ:
Among other mistakes, this is just one more example of the designers being lazy and not properly describing mechanics that hasn't changed between editions. The 5th edition core rulebook is full of information that were previously on splatbooks, but basic mechanics suffered, leading to a lot of errata removing leftover text from the 4ed core rulebook.