Can the hAP be3 be deployed as an AP, not using router/gateway functions? I think it should, but I want to confirm.
Assuming it can be deployed as an AP, how well does it handle WiFi roaming if I deploy multiple hAP be3 units in my environment?
If we are talking about hAp be^3 media, if you were to put it into a capsman mode aka reset its configuration and just use it as a "dumb" wifi access point and have a router to route the traffic, it would literally act as a point of wifi 7 wifi transmission point.
The roaming should work very good, even with the latest 7.24 there was updates to the MLO FT and stability improvements to make it more precise and reliable.
wifi - improved roaming/steering behavior for WiFi 7 MLO;
wifi - upgraded wifi-qcom driver;
I personally don't have the device but its a unified ecosystem and i find everything thats in a capsman deployment that i deployed worked very well.
TLDR; if you buy 2-3 of them and have a router that would be Capsman controller, add them to the pool of AP's configure their profiles and it should all work seemlessly. I don't see people complain on forums about the wifi of it so..
hAP
Nomen est omen
Is it also possible to use the hAP be3 as a "wifi switch"? Just network vlan traffic routing and wifi between clients and a firewall?
You can run the devices as you please, i have cAP ax's and i can run my whole network on just one of them, will it be optimal? no. Should you do it? no.
Hap be^3 is a bit different, i think its meant to be a all in one router/switch/wifi ap.
But as i said, you do whatever you please and use it however u want to.
without external antennas ?
with beefy CPU
and 2.5G ports which draws considerably more power than 1G
and bunch of unused ports & functions ...
nice deployment 
It can absolutely be deployed as a standalone AP (just switching) or, as OP asked, as a part of what other manufacturers usually refer to as a mesh (where the APs are connected with wired ethernet for backhaul and the clients are supported in roaming between them.)
However I think the intended use case for this model is for it to be a central router/AP combo unit, and have its wifi coverage extended by other units. Probably the best one for this currently would be the cap ax or the hap be lite. None of these match the wireless capabilities of the be3, and we can only guess that a cap be should be coming along.
The AP market is fairly well saturated with other manufacturers, and so it might be worth considering non-Mikrotik options. Mikrotiks really stand out when you want complex configurations or routing arrangements, but their wireless offerings don't really have much, if anything, to provide that their competitors don't.
i literally said that 
I just repeated it to soften my main suggestion: look elsewhere.
180 bucks or so each multiplied by the3-4 device used Is a kind of budget that does offer alternatives.
Yes, it will work, but you are wasting a lot of resources if you use it as AP only.
I would wait for Mikrotik to launch wifi7 wAPs and cAPs.
If you must have wifi7 now I would also consider other options. Mikrotik is sadly lagging behind other vendors when it comes to wifi.
Yes, it can be done, RouterOS won't block you. But I agree with others, wait for a cAP BE.
one can only dream, we just hope their cap be's wont run 80-85C, since the cap ax's already run quite hot 65-73C during hot summer days when the ambient gets to like 28-30C