Background: In January I played a two session heist (D&D) game as a player, two person party. A week ago I DMed a session in Undermountain where one of the player scouted a bit with their familiar, three person party. All were played in FVTT.
While I understand where you're coming from, I think that the mechanics as presented remove agency from the player coming up with a smart and appropriate idea, and replacing it with essentially a 'boring' mechanic.
Running a Heist game is quite hard with a D&D party if you want something different then your average dungeon crawl. Not only does the DM need to think differently, so do all the players, which is also difficult for the players. What the player wants to do is exactly what you need in a heist, so why punish the group/player for that?
In our heist game we had a couple of stages: Leg work, prep, and infiltration. Leg work for example was not always as detailed, we broke into the builders guild (for blueprints) without mapping/moving the whole thing, but that was mostly because we had roleplayed bribing the guildmaster earlier and getting details on where to look. Things with arcane eye and familiar scouting also happened. We were a two person party and we had the patience for each other to let each one do their specialty.
With the familiar scouting last session, while it was one player who could do that, it was essentially the whole group interacting with the familiar scout. It was reasonably quickly done, not the whole of Undermountain or level was scouted, and the familiar wasn't found out. Think of it as the group moving the party icon over a larger city or area map. I do need to note that I 'cheated' a bit as a DM, to reduce friction and increase the 'fun', was to give them permanent, unlimited range telepathy with just their party members so the group of players had an ingame reason to discuss and play together without limit. That has worked out very well. It allows me to show everything one sees to everyone without restraint and allows others to advise or ask things of others whatever they are doing.
Now, as we're playing in a magical medieval society, you're infiltrating something of import, thus certain folks expect familiar/polymorph infiltration attempts. Thus guards would be alert for unfamiliar animals, and they will be better save then sorry. A housemaid might just swat a rat or bat familiar with a broom and chase away any other animal. The house cat/dog (or more exotic creature) might just kill or brawl with something infiltrating. Imho these would be logical things that happen, IF seen/heard.
I'm against dice fudging and suddenly adding or removing things. But as one of my friends has said, as a DM you're the facilitator of fun. And as such, sometimes you can do things outside your created fantasy reality. If people start to loose interest or the player running the infiltration goes on too long without regard of the rest of the party, there might suddenly be that housemaid, guard, cat or goblin chasing out the familiar... Just make it believable, do it rarely (the best lie is hidden among a hundred truths), and make it good and memorable.
As for a polymorphed scout, just make sure they understand that they are a 1hp creature in possibly a secure facility that could run into a guard, cat, trap, or monster and when that happens when the rest of the party is far away they will be in serious trouble. That might limit their scouting range. Something like a Brain Puppy (Intellect Devourer) has a pretty big detect thoughts range, there might be more monsters like that (which doesn't work on most familiars)...
