The Fuel requirement to do this makes this impossible in any reasonable game between two players of equal skill. Even if it were possible, it doesn't actually provide enough benefit to justify doing because you're directly contradicting Infantry Sections main strength of superior long range damage by removing one of their Bren Guns, additionally, because you're attempting to also be aggressive with this squad you're invalidating the strength of the 1 Bren Gun on approach resulting in a gimped close range squad that doesn't deal enough damage to justify closing in with and a weaker ceiling in the lack of a second Bren at long range.
Not really, there are modes than simply 1vs1.
Unless I'm wrong, you can only get 2 weapons on the Engineers, both the Recovery Sappers and normal Sappers. Sappers do become durable at Vet 3 but at the point where they are Vet 3 and you've placed double Vickers K's on them they are no longer a supporting Engineer unit. You are using them as a Mainline Infantry Unit and that is okay. When compared to Infantry Sections they're just fine and you're trading a late game infantry performance boost for an early game performance deficient when compared to Sections, not only that, but you're denying the ability for handheld At and Minesweepers for the ability to do this which is a fair tradeoff.
You are wrong heavy sapper upgrade give up to 3 weapons plus armor.
And are not mainline infatry. They are cheap unit with low reinforcement cost and tonnes of utility.
Putting Brens on Raid Sections make no sense as you can simply build regular Sections and place Brens on them for infinitely more impact and better performance. The only justification here I suppose would be the ability to skip Bolster which would leave you with a weakened Tommy + Sapper for not much benefit. While again placing Bren+VickersK on a Raid Section runs into the same problem as with regular sections that you're taking a unit designed with a purpose in mind and neutering that purpose for general utility which is shown to not work because utility does not trump specialized strength.
Your fears are unfounded in any practical sense.
Again not really, its like saying that putting lmg on Riflemen makes no sense. LMG on Riflemen work great.
It actually does. They can fight longer range unit better and still be good at mid range. In sort the unit instead excelling in one range it becomes good at all range.