Some thoughts for you.
"Economical" in this instance may infer a % of capability which you may be happy to accept but others may not so I don't believe one design fits all in this area. 100% availability may never be totally possible but one can come close depending. The question then is what availability are you expecting?
Primary effort has to be power supply, batteries, solar & regulator etc with 1 of each being a single point of failure especially with a combined MBPro and USB modem. Split the modem and MBPro unit/power wise and the capability then exists to manage the power, that is if your battery is going down for some reason, say failed panel, regulator or no sun for extended period, all tied together with battery capacity etc then with a separate Modem one could auto switch the modem off while giving the MBPro longer run time while maintaining data/logging etc, of course up to a point where the MBPro would need to be auto shutdown to protect the battery.
Once the modem is shutdown then this can become a flag that something is wrong and gives a chance to rectify the condition without loss of data etc as the MBPro will still be running, for a period of time anyway? A combined unit once powered off is 100% loss for the time involved.
A combined MBPro/USB modem could draw 500-600ma which is getting up there without a fairly hefty battery & panel to provide some backup contingency. Another thing to consider with a combined setup is remote access to the SMB data area, that is if you require such access?