I'll try to summarize my view of spare HQ sub-topic.
1. I haven't thoroughly tested relevant game mechanics yet, but from Steve posts (quoted above), there must be no advantage in commander survivability under fire by adding them some spare full-sized HQ elements, because commander KIA chance is determined by their HQ element size. Doubling formation HQ size by adding spare HQ elements of the same size is doubling probability of HQ formation being chosen as target and being hit by deadly fire, and, simultaneously, halving commander's KIA chance by any hit, so 2 x (1/2) = 1x chance multiplier, that is no advantage at all.
1.1. It must be considerable (and even drastic) advantage in commander survivability under fire by adding them midget spare HQs ("command shelters"), though those shelters will not work as effective command infrastructure (commander bonus delivery). If it's exploit - that's a question. It's too cheap, nearly zero cost, but field shelters are not very expensive things. It seems to me as good idea, if you have a home rule to not use them in too much numbers. It can be strong exploit if this "shelter" will provide superior bonuses transition to subordinates, as if it is working combat communications infrastructure instead of cheap tiny shelter.
1.2. To minimize commander's KIA event probability w/o exploits, you have to give them absolutely minimum HQ capability (main HQ element size), that is necessary to provide their full command bonuses to their deployment+subordinate formations. Bigger main HQ element - higher chances of HQ formation being chosen as target. Design main HQ elements as small as it can be without deficit of command capacity to their purposed ground force size. Add command shelters, as described above, if you have no RP objections to this idea, but do no opposing thing: do not expand HQ element capability more than it's necessary to grab your expected force fully. It's even can be good idea to reserve front-line elements (supernumeraries), because they will suffer losses during first phases of battle, and your force size will drop to smth like mean size at HQ capability volume. It can be effective doctrine to count on mean force size during battle, not fresh-and-untouched one, that will be so in ideal circumstances only.
1.3. Don't forget to click "Avoid Combat" checkbox for all HQ elements - it's decreasing chances of being chosen as target greatly, with no described harm to command abilities. I haven't tested if "Avoid Combat" is halving arty's effectiveness, as described in Steve post generally (without mentioning support fire), but if it is - arty components still have their halved effect, so use them in HQ elements if you have no desire to reserve secondary vehicle sockets for AA/FFD/CE components.) I haven't tested too, if opening support or AA fire disables "avoid-combat" status for this element during this combat phase (it will be realistic, but Steve haven't described it, so it can be little exploit).
2. Multiple full-sized spare HQs, being questionally-effective as commander's preservation measure, will be tonnage-coslty inversely proportionally (non-linear) to your force size.
2.1. For topic-starter's force sizes (Marine Company to some temporary composite battalion, combined from several such companies) proposed 20x or even 10x are very costly, it will be thinning front-line segment of you force greatly, that's very bad idea. 2x spare infantry HQs can be used in this level without considerable drop in combat effectiveness.
2.2 For regiment to brigade-size (20 to 100k) 3- or 4-tier overall forces, multiple full-sized spare HQs in upper and middle tiers are not so much tonnage-costly, though very questionally-effective even with very good commanders, if there will be any positive effect at all. 2x spare HQ elements, even vehicle ones, can be used in this level without considerable drop in combat effectiveness.
2.3. For large divisional to army sizes, upper-level HQ multiple redundancy can be nigh-negligable in terms of tonnage saving.
2.4 If HQ redundancy mechanics will work as intended, it's higher-level better tonnage-costs must be considered with caution, because higher-tier commander bonuses will affect combat troops an masse with lesser bonus transition multipliers.