To address point 1: Well, as a profit driven entity, the MRBC probably charges a surcharge and retainer fees to the Houses for holding the contract money in escrow on behalf of the contracting parties. It would stand to reason that since the mercenary units in operation, the more money the MRBC can generate, that they would provide loans at probably fairly reasonable rates to help merc units out in a pinch.
They do indeed, per FM: Mercs Revised rules. Quote below.
FM: Mercs Revised, pg. 169
For contracts negotiated and agreed upon through the
Mercenary Review and Bonding Commission (on Outreach and
Galatea), a 5-percent deduction applies immediately upon
acceptance of the contract by both sides of the negotiating table.
This deduction is the handling fee imposed by the MRBC, which
places the remainder in trust pending the final completion of the
mission.
As for the loan part, I'd give two suggestions.
1. The "reality" suggestion - The share of profits from the Dragoons, return on investments from Blackwell, taxes on goods/services on Outreach, MRBC revenues from training fees, contract processing fees, etc; these things are shared. The Dragoons like to maintain the image of squeaky clean and all three (Blackwell/MRBC/Dragoons) being separate, but there would very likely be a great deal of behind the scenes moving of money to communal funds. That means a lot of potential money to loan out, which also means interest generating more profit on those loans. And even better, if you default, you've just irked off the MRBC. Bye-bye reputation. As one solo owner of the premier place for mercs to call home base, they've got the corner on a fairly ugly, dirty business. And they are filthy rich from having that corner. This fits in better if you're stepping further away from canon, as it can quickly break the existing canon paradigm.
2. The "FASA" suggestion - Fasanomics rule here. Even with all the potential for revenue and combining income streams, the Dragoons are effectively only breaking even. Any profits go into reinvesting in Outreach, Harlech, expanding the MRBC, or the Dragoons. Sure, they have a large warchest, but that is for usage by the Dragoons and emergencies. They're not building Outreach into the kind of place that could rival Terra for quality of living, they're not holding onto their more socialistic style economics as found into the Clans, and incorporating it on their home planet. (A home planet that could be a virtual paradise in the Inner Sphere.) They're keeping all their funds separated, they're squeaky clean, and they are some kind of austere warrior-monks from the Clans that apparently have no desire to better their people's lives or enrich themselves. FASAnomics all the way here. The Dragoons, the Kell Hounds, the Highlanders, and all the rest are somehow not filthy rich, nouveau nobility that could make their home planets (and even surrounding planets) A+ worlds in no time flat. This fits best with things hewing closer to canon material.
As klinktastic mentioned on point 3, wealthy investors don't happen a great deal. There is some canon precedent for it, but more often in it is the vein of "become my household guard." This can be seen with a few House units, as well as mercenaries working for Duke Ricol of DC fame and others. The only exception I can think of is Lethal Injection (FM: Mercs Revised), who was invested in by Count Kit de Summersville from the Magistracy. How he intends to turn a profit or see a return on investment? Got me. FASAnomics again. First time they salvage a unit, the Count should take that as his claim, sell it, and call it a day.
