4:102 itself says what to do so that the enemy does not rush upon you in one go...
Salaam Wakas. I am sorry to say it, but you are terribly mistaken about the rerason why they need their weapons ready even during their صلو in that life and death situation. They need them in order to defend themselves when the enemy attacks. The enemy will attack no matter what.
Please refer to 4:104. The attack did happen and the believers had such terrible time that "the land, for all that it is wide, did constrain you, and ye turned back in retreat." (9:25)
You can never expect professional commanders to take you seriously if you insist that an army may do without rear and forward teams to look out. They do their best to discover the whereabouts of the enemy but they may still fail. In the case we are discussing they failed.
4:102 ...and thou uphold/establish the salat/bond for/to them*, then let a group from among them stand/establish** with thee and let them bring their weapons; then when they have SaJaD then let them be behind you (plural)...
*can be translated as for/to/unto them, e.g. see 2:109, 5:75
**again, it can mean "stand" figuratively, e.g. see usage in 4:127
Please excuse me again but from these words I cannot decide what you really think is the صلو that verses 4:101-103 deal with.
a Quran lesson,
a briefing on the battle tactics as a friend here has suggested,
an attempt to boost the morale of the believers in that life and death situation,
If I knew your idea, I would be able to tell you whether it is plausible or not.